1/16/19, PM Shift
A winter storm is still expected to produce a widespread snowfall Sat night thru Sunday across the North Country, followed by the coldest airmass of the winter season. Our forecast area continues to be on a sharp northwest to southeast qpf/snowfall gradient and a 15 to 25 mile shift either north or south will have significant impacts on anticipated snowfall and precip type.
As the system continues to become better sampled over the next couple of days, the details and especially snowfall forecast will become clearer. I agree with the previous forecast of a general widespread 6 to 16 inches with localized less amounts toward SLV (4 to 8 inches) and higher amounts possible toward VT (10 to 16). Another challenge besides sharp qpf gradient is snow ratio`s given the very cold low level airmass and expected moisture and mid level temps profiles.
Water vapor outlines our upcoming system nicely over the eastern Pacific, while deep sub tropical moisture is advecting already of deepening western Conus trof. Interesting our short wave of interest is located near 150 W and will quickly eject around closed circulation near 140W and 40N into the western Conus, before lee side cyclogenesis occurs on Saturday. GFS/ECMWF/GEM and FV3 all track this potent southern stream energy into the MS River Valley by 12z Saturday, while 1000mb low pres is located over western TN. Meanwhile, strong 1038mb high pres near Hudson Bay will be driving a very cold and dry low level airmass into northern NY and VT on Saturday, per falling 925mb temps btwn -20c to -24c range, setting up the low level moisture/temp battlefield across the ne CONUS.
1000mb low pres will ride along this low level thermal gradient toward southern New England, while deepening to 992mb or so by 18z Sunday. 12z guidances continues to show open 7h/5h circulations, while 925mb and 850mb have several closed contours progged. As in previous years open waves at 7h/5h tend to produce sharp nw/se qpf/snowfall gradients with models typically overdoing the northern extent of heaviest qpf. In addition, models progged pw values range btwn 0.25 to 0.50 across our cwa, which is only a std or two above normal, while values approach 1.0 over southern New England. These factors support qpf ranging from 0.25 to 0.50 slv/western dacks to 0.50 to 0.85 central sections to 0.75 to 1.25 southern zones. As far as snow ratios given good deep layer moisture in favorable snow growth/omega couplet I would think 18/20 to 1 for a 6 to 8 hour window, with slightly lower on the beginning and end due to limited moisture/lift. This results in snowfall amounts of 4 to 8 slv/western dacks, 8 to 16 inches eastern dacks into VT, with local variability and subject to change.
Soundings show all snow profiles for majority of our cwa, however latest 12z gfs at VSF shows warm layer btwn 890mb to 770mb of 1 to 2c, supporting some sleet potential. Once again the thermal gradient is very sharp and a slight change in track will impact ptype. Will continue to mention all snow in grids, as threat for sleet is still minimal attm in our cwa.
Jet structure still looks favorable with anticyclonic curved 170 knot lifting over southern Canada, while developing 850mb southerly jet of 45 to 55 knots is moving across the mid Atlantic States and nosing toward our southern cwa. This jet couplet structure, along with strong 850 to 700mb fgen forcing btwn 06-12z Sunday will aid in deep layer lift across our most of our cwa. By 18z Sunday, developing northern stream energy tries to phase with southern energy and advect additional moisture back toward our region, but feel this occurs to late to have significant impacts. This energy will help to deepen sfc low pres over eastern Maine, while brisk northwest winds and strong low level caa develops.
For temps, 1038mb high pres advects 925mb temps btwn -20c to -24c by 12z Sat with some northern gradient. Lows generally in the 0 to -20f range, with coldest across northern NY. Depending upon gradient flow, some wind chill highlights maybe needed. On Saturday, 925mb temps warm only 2 to 4 degrees with developing clouds, support highs 0f to 10f. Temps with clouds/precip developing on Sat Night hold in the single digits to near 10 above. Very cold and developing northerly winds occur on backside of sfc low pres for Sunday with highs ranging from single digits slv/northern CPV to upper teens/lower 20s vsf, but falling. The core of the coldest temps arrive on gusty northwest 850mb winds of 40 to 50 knots on Monday aftn/night as progged 925mb temps plunge btwn -25c to -28c, before modifying on Tues/Weds. Looks like an advection cold on Monday night, with wind chill highlights expected. Lows mainly in the -10f to -25f range with much colder wind chills. Temps rebound quickly as southerly flow develops Tues/Weds ahead of our next system. Trends are your friends if you like a colder and more wintry precip system.
Confidence continues to increase for a significant multi-hazard winter storm impacting eastern NY and western New England Saturday into Sunday. Main hazards include heavy snowfall amounts and rates, possible period of sleet/freezing rain leading to ice accumulations for some(mainly Catskills, mid-Hudson Valley, NW CT) followed by strong winds/blowing snow potential and an arctic air mass that may lead to dangerous wind chills Sunday night into Monday.
Strong 1035hPa high pressure in Quebec noses builds southward into the Northeast during the day on Saturday with northwesterly flow advecting in a chilly air mass across eastern NY and western New England. High temperatures are expected to remain well below freezing through the region with most staying in the teens and 20s. As upper flow backs to the southwest, clouds continue thickening through the day.
The 12z/16 global guidance has come into better agreement that for a stronger moisture rich southern stream wave originating from the Pacific Ocean will dig into the Gulf States and pick up additional moisture from the Gulf during the day Saturday. As it travels northeastward towards the Ohio Valley and warm air overruns the cold air wedge over the Northeast, snow will begin from southwest to northeast. Exact start time for snow is still uncertain but guidance continues to suggest some light snow is possible Saturday morning from weak isentropic lift before the more appreciable snowfall associated with our winter storm overspreads the region Saturday afternoon and especially after sunset.
Overnight Saturday, our southern stream wave moves into Ohio Valley as a potent northern stream wave associated with a strong upper level low and a very cold air mass over the Hudson Bay drops into the Great Lakes. Global guidance has come into better agreement that the two do not phase until during the day Sunday and this delay would mean our intensifying storm tracks across the I-95 corridor with the associated 850hPa low tracking from eastern PA across the mid-Hudson Valley into eastern New England Saturday night into Sunday. This has implications on the northern extent of our wintry mix/snow line would keep it suppressed further snow to the mid- Hudson Valley/NW CT/Berkshires with areas north and west likely remaining all snow (perhaps brief period of snow/sleet). While the first half of Saturday night likely remains all snow across eastern NY and western New England with high snow ratios (dry/fluffy snow) and possibly high snowfall rates given strong lift extending through a deep column intercepting the dendritic snow growth zone, we could then see the warm nose inch northward towards our area.
The second half of Saturday night looks to experience the strongest warm air advection as our 850hPa jet intensifies to 50-60kts which would warm the column and increase the fetch of Gulf/Atlantic moisture reaching the Northeast. This would then shrink our dendritic snow growth zone and even shift the "sweet spot" experiencing the best lift through the -12C to -18C zone to the Upper Hudson Valley and Adirondacks. While this would decrease snow ratios for areas from the Greater Capital District south and eastward, there is strong agreement amongst the global guidance that a copious amount of moisture advects into our area with potentially 0.50 - 0.75 inches of liquid from 06z - 12z Sunday. In fact, the snowfall band overnight Saturday resembles that of a quasi- stationary snowfall band as described in previous CSTAR research. Thus, even lower snowfall ratio closer to climatology (10 to 12:1) would not limit significant snowfall accumulations and high snowfall rates. Thus, our confidence for areas reaching warning criteria snowfall continues to increase from the Capital District north and west. For areas in the mid-Hudson Valley, NW CT, the Catskills and perhaps the Berkshires, the warm nose could inch close enough the mid levels warm close enough to or even higher than 0C that these areas mix with sleet and even freezing rain. This could lead to ice accumulations as the sfc-925 layer remains cold. Exact icing potential is still uncertain but should be ironed out this event draws closer. The ECMWF is the coldest solution limiting the wintry mix to the far southern mid-Hudson Valley and NW CT with the CMC-NH a bit farther north with the mixing potential while the GFS is the warm outlier. For this update, again went with a ECMWF and CMC-NH blend for precipitation type.
By Sunday, our northern stream and southern stream wave phase over the Northeast allowing our positively tilted trough to become neutrally titled. Snowfall continues into Sunday morning with cold air quickly wrapping in behind our system with our warm nose quickly shifting into New England. Any wintry mix Sunday morning should then transition back to all snow during the day with snow ratios increasing again. Winds during the day Sunday look to also increase as the storm matures over New England with gusts up to 25-30 mph possible potentially leading to blowing snow. Snowfall should linger through most of the day Sunday before exiting from west to east late in the day into Sunday night. Total liquid equivalent for this event remains impressive with very good model agreement with 1 - 1.75 inches of liquid across the region which is why we continue to believe this event will lead to significant snowfall amounts. Very cold air continues advecting into eastern NY/western New England Sunday night with 850hPa isotherms dropping to -19C to -28C and surface lows falling into the single digits and even below zero from the Capital District north/west. Strong winds continue overnight as well with gusts 25 - 35mph possible which could lead to dangerous wind chills and necessitate wind chill headlines.
High pressure builds into the region Friday night into the first half of the day on Saturday, resulting in a brief period of quiet weather ahead of the next system. Lows Friday night will range from the upper teens across outlying areas to the upper 20s in the New York City metro.
By Saturday morning, a southern stream trough and its associated surface low will begin to eject out of the southern plains, while at the same time a northern stream shortwave begins to dive south out of Canada. Given the time frame, there is reasonably good agreement between the various model solutions with the low tracking through the Tennessee Valley, then passing near or just southeast of New York City during the day on Sunday. However, given the position of the low as it emerges off the coast, small changes in track will have significant implications for the type of precipitation observed across the region.
In terms of sensible weather, clouds will increase through the day on Saturday ahead of the system, with overrunning precipitation reaching the area by late Saturday afternoon/early Saturday evening. With daytime highs ranging from the upper 20s to mid 30s, temperatures will initially be cold enough for precipitation to start as snow. As southerly flow ushers in increasingly warm air Saturday night, precipitation will transition from snow to rain from south to north beginning around midnight and continuing through daybreak Sunday. While much of the area will see rain during the day on Sunday as temperatures rise into the 40s, far northern portions of the Lower Hudson Valley could remain cold enough for some mix of snow, sleet, and freezing rain to continue through the day. The type of precipitation that falls during the event will be strongly dependent on the exact track of the surface low. A low position slightly farther south and east, like that of the operational 12Z ECMWF, would bring significantly more snow to the region.
As the low moves off to the northeast on Sunday, cold air will rapidly rush into the region, changing any lingering precipitation back to snow Sunday afternoon. As temperatures rapidly drop back below freezing, any lingering standing water could quickly freeze, creating hazardous travel conditions. Temperatures will continue to plummet through the overnight hours, with lows Monday morning ranging from the single digits to lower teens. Gusty northwest winds will result in wind chills 5 to 10 degrees below zero.
Thereafter, arctic high pressure builds into the region through the first half of the work week before drifting offshore Tuesday night into Wednesday ahead of the next system approaching the area.
Saturday night and Sunday... Phased-stream flow brings higher moisture amounts to this system. Cross sections again show deep moisture. Precip water values have diminished a little, but still show values around 1 inch feeding into southern and eastern parts of our area. These PW values are well above normal in mid January.
Still uncertainty in this forecast as the models continue to shift. Most confidence is in the ECMWF and allied models which have been the most consistent in showing a center track along or near the South Coast. The GFS solution has shifted toward the ECMWF. This is a colder solution, and supports more snow and ice in the forecast than previously.
Based on this, the forecast starts with snow everywhere Saturday night, then brings rain into the South Coast by morning. Rain spreads up to Boston and the North Shore Sunday, also across RI and much of Nrn CT. North of that zone will be a band of sleet/freezing rain in the interior...and snow in parts of northwest Mass. Shifts in the forecast storm path north and south will of course shift these precipitation bands north and south.
Strong east winds will reach 35-50 mph speeds along the coast, especially over Cape Cod and Islands. Inland speeds will be more like 25-35 mph.
Sunday night-Monday The storm passes our area between 1 PM and 7 PM Sunday. Winds will quickly shift out of the north and northwest. Strong pressure gradient at the surface will generate strong sustained winds, and cold advection will draw strong winds aloft down to the surface with gusts of 25-40 mph possible. The combination of wind and cold air will bring wind chills to single digits and below zero Sunday night and Monday.
High Impact Weather:
Significant winter storm will impact the region from Saturday night into Sunday.
Overview: Confidence is increasing for a significant winter storm to impact the region over the weekend. Saturday will start off clear and cold over northern New England as high pressure exits to the east. Meanwhile our storm will develop over the four corners and rapidly intensify as it moves across the southern Plains. By 00Z Sunday we`ll see the first flakes moving into southern New Hampshire as the low moves up the Ohio River valley. By Sunday morning the low will move offshore of Delmarva with significant snowfall in the northwest quadrant. Heavy snow will continue through the day on Sunday before coming to an end late Sunday night.
Trends: Over the past day or so we`ve seen a consistent colder trend in the guidance as the low center shifts further offshore. As of 12Z a super majority of ensemble members would keep the low center east of the Maine coast over the Gulf of Maine keeping our region in snow. Upper level pattern: This nor`easter will form in a pretty classic pattern along the eastern edge of a deepening 500mb trough. To our west the west CONUS remains under strong high pressure with the result being blocked flow. This allows our storm to source from two regions. A short wave in the northern stream will dive south through the Canadian prairies. As it does the southern stream low over the southern plains will also phase with the two systems combining to for a very elongated low pressure along the coast. Whenever there are two systems trying to act in phase there is room for error, and with neither of them onshore yet that is certainly the case here however the presence of an upper block over Alaska along with weakly blocked flow over Europe supports the strengthening east coast trough.
Ptype: Over the past few days ptype has been a concern with this system but it gradually becoming less so as the colder trend appears to be winning. The key here is really that northern stream short wave will provide the forcing to keep the warmer air offshore. Trying not to flip flop too hard have opted to leave some mixed sleet and freezing rain in place along the coast but feel that an all snow solution is becoming more and more likely. A bit of freezing drizzle is also possible on the backside in the dry slot but overall we are looking at a snowstorm with a chance for some mix along the coast not a widespread icing event.
Snow Amounts: If you`ve been waiting for winter, here it is. Potential for very heavy snow accumulation /widespread double digit snowfall/ continues to increase. With 1-1.5" of liquid precipitation, even a modest 10:1 snow ratio easily achieves that objective. In fact higher snow ratios are quite possible, particularly in the interior. Forecast soundings show strong upward motion in the snow growth zone on Sunday morning across southern Maine and expect a region of snowfall rates over 1"/hr through Sunday. It`s far too early to pin down any mesoscale effects other than to mention that strong snow bands are likely with this kind of strong dynamic storm and any areas that remain under snowbands may see closer to 2"/hr rates.
Wind: A look at the pressure gradient and rapid strengthening of the system says that strong winds are likely with this system. The trick will be getting them down to the surface as a more stable cold layer over land may limit the mixing somewhat. However along the coast line and certainly over the coastal waters stronger gusts are likely.
Lighter winds and high pressure will allow temperatures to drop well below zero Saturday evening before clouds thicken later in the night. A massive area of low pressure will track out of the southern Appalachians towards the New England coast by Sunday morning. This developing Nor`easter will bring an impressive swath of southern stream moisture northward, overrunning the bitterly cold Arctic air mass. Initially, snow ratios will be very high...before winds pick up and the warm nose arrives. Used a multi-model ensemble mean for temps Saturday night into Sunday night. The very cold low level wedge of air will remain in place with unusually cold temperatures to go with moderate to heavy snow. Current forecast wind chills on Sunday meet advisory criteria in northern zones...something that is difficult to recall in the past decade. Our current storm track takes the low across the Gulf of Maine towards Saint John, New Brunswick and keeps the entire area in snow until late afternoon when sleet arrives into the Down East region. Intense banding with 2 inch an hour snowfall rates are a good possibility and overall totals could reach 15 to 20 inches towards Bangor...tapering towards a foot in northern Aroostook County. The warmer GFS solution remains a concern...reducing snowfall for Bangor and southern zones and potentially creating a corridor of significant ice and sleet. Winds increase during the afternoon and blowing snow will be a big concern. Between snowfall rates and blowing snow, travel could be quite dangerous at times on Sunday. The dry slot arrives towards the southern zones Sunday evening, but the trowal will likely keep the snow going in northern zones through Sunday night. Blowing snow will continue into Sunday. Lighter snow will continue into Monday until the upper trough passage. Temperatures will actually moderate after the storm on Tuesday and a warm advection event may generate some snow Tuesday night into Wednesday.
One other detail regarding the Nor`easter could be coastal flooding. Right now, the Sunday high tide looks like it`ll occur before the waves really ramp up later Sunday afternoon...and winds will be offshore by the next day`s high tide. For now, it looks like it`s a near miss, but will have to watch whether the storm accelerates and aligns better with the Sunday high tide.