wdrag

12/27 620AM Topic Update. Evolution of this topic continues as 12/29 is gone (except a few I-84 snow showers) . This includes dropping graphics for 12/29 as 12/21ish modeling completely missed this non-event for our area. New graphics will be added Monday morning as modeling consensus grows for 12/31-1/2. This system for 12/31 and especially Jan 1-2 is trending toward broad high impacts, ranging from at least a period of advisory or warning ice and possibly advisory snow for the I84 corridor (high terrain above 1000 feet especially), to a multiple slow response river flood response (minor to moderate flood stages in our NYC forum), depending on new qpf (not including any remaining snowmelt) and how much of that qpf is ice and remains ice through Jan 2. In my mind this is complex. Scenario is a weak northern stream short wave passage on the 31st dragging a cold front through the NYC forum accompanied by rain, (possibly a little snow or ice I84 high terrain). It's possible NY eve celebrations will be precip free for a few hours? Meanwhile the southern stream short wave may split into two pieces, leaving something behind for the 3rd, but of little or no impact here. It's the preceding newd lift out of the Gulf States short wave that is going to be a potential high impact player here on the first and second of January. How this plays out is uncertain but clearly the 00z/27 GFS and EC op have trended colder while the run to run inconsistency of the UK and ICON continues. In essence though...it appears to me that a warm front to our south will provide the necessary lift with a southerly jet of 50kt (PWAT 1") impinging on it, to dump heavy qpf on a large part of the NYC forum, on the order of 1-3". I am tending to lean widespread heavier than 1.5". That front may eventually lift north of LI/I80 but I think there is a chance of an occlusion and waves of low pressure moving out to the s of LI. That leaves potential for heavy icing parts of I84. So, tbd, if this read is generally reasonable but worthy of comment-updates. Wind: 50+MPH gusts...still possible for LI/NJ but less likely. Have left the Topic TAGS as is, but updated the topic headline. Basically done w this for 24 hours. Thank you for your updates-impressions of the modeling. I know this differs from the GFS parallel (not necessarily a desired model upgrade yet, in my mind, especially if it misses this qpf event down here-remains to be seen but I'm concerned about consistently better GFS V16 performance vs the operational V 15). Ice, snow and wind support for my expectations above are derived from 00z/27 GEFS probs and operational model trends.

Good Sunday morning everyone, While there is still opportunity for a short period warm-wet windy scenario for the NYC forum Jan 1, this large scale long duration complex event is trending colder icier for I84 and definitely heavy qpf-exacerbated flood potential, as the southern stream separation and northern stream deamplification short waves continues. Topic updates ~ 7AM.

Could be windy warm rain... don't think we'll all have a firm handle on this til Monday morning. Just too many solutions. EC is still warm, not as wet as GFS ,and quite windy, but less northern stream amplitude.

Ice? we'll see have to ride it out for 36 more hours of model cycles, i think.

Something is up for 1/1. 12z/26 UK op w snow to PHL and southern stream is out in piece and weak(let's keep in mind the UK is a wildly shifting model imo). 12z/26 GEFS ensembles showing southward signs including snow to I84. ICON op is south with temps mid to upper 30s n of the warm front NNJ-CT northward. Not yet a done deal and options seem to be opening up a little more, beyond the windy warm wet scenario. Have to ride it out and I probably won't be back on til this eve.

Here's some more review material: Wind power outages leftover more than a day after the fact. NO snowmelt qpf analysis (general by radar-platforms) which has resulted in many minor small stream flood events. Impact is mainly local detours and flood plain basements/yards. Probably my last on this. Top Areas by Outages Tennessee 22,208 New York 14,596 New Jersey 6,612 Pennsylvania 5,521 Florida 5,411 Last Updated 12/26/2020, 07:32:29 AM

Good day after Christmas everyone! I have adjusted the previous 12/29, 1/1 topic and caved in on 12/29 being a non-event for us. But 1/1 is coming and still one for me with modest winter weather interest for 1/84. Here is from the topic edit. 12/26 612 AM edit. Ensembles (GEFS, EPS, NAEFS) favor a primarily warm wet windy solution for early Friday-NY Day, but thereafter there is some uncertainty. UKMET and to some extent the 06z/26 GFS are suggesting a separation of short waves across the eastern USA with a trailing southern stream significant short wave rotating northeastward as the NY eve cold front is draped somewhere across our forum, possibly even down to the Mason-Dixon line. The UKMET op is an erratic model swinging wildly at times and at 120 hours, am a little reluctant to put much stock in its suggestion. Still, other models are suggesting a more sw wind at 850MB into early NY Day which tells me that the northern stream short wave is not initially digging into the Ohio Valley this time, as per this most recent damage/flood event. However!!! this is a low probability chance but one that I do not want to discard, yet, till I see all models wet-warm-windy with little or no snow behind it, across I-84. That leaves us with a 1-2" qpf event mainly 6PM NY eve to 3PM NY Day, most of it probably in a 6-9 hour period. This would exacerbate-extend the already projected minor-moderate flooding on some of the NYC sub forum rivers, and probably result in even higher stages and a few more streams-rivers into flood. Regarding wind- recent model cycles are offering a short period of 50-55 MPH southerly gusts NY eve night or thereabouts. GEFS and EPS 850MB winds after already over 50 knots. Operational models suggest 850MB winds 80 knots. This event may end up 5-10 MPH less than that of early 12/25, but it could be a bit warmer (more sw wind at 850MB than due south) in some areas, so that downward momentum transfer is slightly better (less of an isothermal boundary layer). With ground already softened by recent rains and snowmelt, and largely unfrozen by the time of the coming Thursday-Friday event, this could yet again result in power outages from limbs and trees down. So, some things to ponder, at least for me. I'll adjust this topic by sunrise Monday, once the event evolution becomes solidly wet windy.

Good Saturday morning Matt and Don, and I hope a continued good holiday season for you and your family-friends-- saw the GFS (00z/06z 26 version). Right now, GGEM/NAEFS not much sign this far northwest but it could adjust and become significant. I am a bit more interested in 1/1 for I84 as GFS is sagging and UKMET a bit more separation and will adjust topic accordingly (still warm but UKMET, the erratic supposedly #2 model in the world) is separating streams and offering a weaker solution 12/31 and a southern streamer late Jan 1. For now will go with the preodondernace of warm 1/1 and await GGEM coming on board with the GFS, ditto EC for 1/4ish. Myself I am quiet and not chatting beyond 6AM Jan 2. fwiw... exit 8 I84 (Mt Cobb at 2000 ft) had a least an inch last evening. will check cocorahs at 10AM.

Hi everyone, FFG will be down quite a bit for the 12/31-1/2/21 event. Wind guidance currently favors 5 MPH less than what we experienced this morning. Want to see 00z/26 model cycle, in particular if any separation between the northern and possibly dragging southern stream short wave, which then would make things considerably more interesting for wintry precip. As of now, continue the warmer wetter windy idea with no model support for my wintry concern. So, tomorrow morning, if it looks solidly wet, I'll work on changing this thread (preserving the original poor 12/29, 1/1 wintry impressions). Happy Holidays! Walt

Please see bluewave posts little earlier on temps 61 or higher with big wind and then less wind where temps basically peaked in the upper 50s. Temperature profile makes a big difference,

Merry Christmas 2020! I will revisit this topic either late this afternoon or Saturday morning for a change in title featuring Jan 1--- but only if the EC is consistent at 55 + MPH gusts. Anything less, becomes our routine event. Something is coming Dec 31-Jan 2 and it may be more complex than what we see right now ranging from some sort of wind-flood event to a system with a very low prob of some wet snow. I kind of want to be careful about a topic adjustment, if any. For now consider the significant snow threat non existent though the GEFS,maybe by climo or whatever, has something for at least I84 on the first.

Saw the upper 50s temp cap on wind below 40 kt at ISP/GON so far, Also the 50-55KT at JFK/LGA/BDR. Not sure what CP max temp was--probably only 61ish. Also bluewave pointed out the MOS warm temps vs GFS raw were wayyyy too high for e LI (good catch). Am wondering if that contributed to the GEFS high bias probabilities for 60+ MPH gusts eastern LI? Merry Christmas! Walt

Isothermal layer probably prevented much more impact on LI..s Coastal CT... still fairly impressive in the area at 510A. Here are some platforms for cursory verification in a quick check.

47kt Tappen Zee light, 49kt Ocean City Maryland and 46 kt at Lewes DE. Coming up. should be blowing good midnight-6A LI. Might not be so bad nw of I287. Even Vernon NJ high terrain 53 MPH recently. Probably off line now thru 5A.

Suggestion: Do NOT use the NAM 10M gusts as what will occur. They look 10-15 MPH too high already. I think the HRRR is best.

Widespread pure drainage flooding can be expected along the edge of the snowpack in our forum overnight, anything 6" to a T.

thanks for the post. Vernon NJ (elevated ski area I think) hit 47 MPH this morning.

Let's temper that 10-15 MPH, and use that 80 for an extreme gust possible but not something I'd want to forecast. I'm staying with 5s5-65 MPH gusts with a few 70-75 MPH somewhere NYC eastward and northeastward. NAM can get a little ballistic. Still...it's potent, especially if lines cross the area 5z-12z.

I'll look for your report. Unusual flooding or debris, let the forum know and NWS as well. Good work/call etc.

Agreed with your concerns in heavier snowpack on northwest fringe of NYC subforum. Already well in advance flood "warnings" posted for PA/NYS Rivers in and near our subforum, showing confidence on the coming event.

For the record as a starting point: 11AM today. No significant outages eastern USA.

FOUS 29kt at LGA and 28KT at PHL says to me 40 kt gusts possible in NJ beginning around 5PM. I dont want to overplay this but my experience with BL winds of 27kt or greater is a wind advisory. 34KT or greater HWW. So 5P could start 40 kt gusts. Also, once the pressure dips below 1000MB early Friday, the 1000MB wind is easily the guide for minimum gusts..possibly sustained. This should happen NYC north and northeastward. So if you have model 1000mb winds at 09z/25...check that too.

Haven't had time to study... Christmas doings here. Just check HRRR and others on here will have time to assist.

Hi! I'm not well educated on the subject. I ask that you google Richardson #, also BRN. From FSU this morning... LGA 12z/24 3K NAM winds and R#. You can see R# allows up to 950MB... but 50KT is modeled just above the sfc. Hope this helps and works pretty good as predictor. Here's a snippet. I look at SOLID red 0.25 surface to whatever layer and grab the max wind in that layer and trim 5 to 10 KT. In the limit of layer thickness becoming small, the Bulk Richardson number approaches the Gradient Richardson number, for which a critical Richardson number is roughly Ric= 0.25. Numbers less than this critical value are dynamically unstable and likely to become or remain turbulent.[1] The critical value of 0.25 applies only for local gradients, not for finite differences across thick layers. The thicker the layer is the more likely we are to average out large gradients that occur within small sub-regions of the layer of interest. This results in uncertainty of our prediction of the occurrence of turbulence, and now one must use an artificially large value of the critical Richardson number to give reasonable results using our smoothed gradients. This means that the thinner the layer, the closer the value to the theory.[2]

At 10PM, our NJ forum should be starting to get reports of 46-55MPH (wind advisory) gusts and isolated power outages-same e PA and w NJ. I trimmed these winds from Knots down to MPH.