arlwx12

Slightly updated wording from LWX's AFD at 1029... If the upstream ridge-building event takes place a bit further west near 120 W longitude on Saturday, downstream trough amplification would be favorable for phasing over the mid MS Valley into the OH/TN Valleys Saturday night. A subsequent low track hugging the coast would become more likely in this scenario, aided by downstream blocking and a potential source of cold air given the upper lows over Maritime Canada and surface high pressure attempting to ridge into northern New England. There is also an appearance of an inverted trough signature in a subset of model guidance over the last couple cycles, given interaction between a more amplified northern stream wave and a bit more distant offshore southern stream low. This could factor into precipitation amounts and placement. After reviewing the 00z model guidance suite, some subtle consistency has been noted although subtle uncertainty remains. Most of the models has a low tracking up from GA/AL toward the NC coast and off the Delmarva coast during the late Saturday night into late Sunday timeframe. The question remains in the proximity toward the coast along with cold air availability, and the overall scope of the precip shield pending the placement/intensity of the system. 25 to 50 miles could make a huge difference between seeing little to no wintry precip or impactful precip, especially for those east of the mountains where the confidence remains low due to thermal issues especially below elevations of 1500 feet. The 00z GFS/GEFS solutions have come down a tad, but still produce measurements on the order of 10+" of snow over a large chunk of the area. Meanwhile, the 12z EPS ensembles came up some, but at much more conservative levels which align with deterministic runs of the NAM, RDPS, GDPS, GEM, CMC, EURO/EURO AIFS, and UKMET. The latest NBM also came up a bit and aligns with the majority compared to the GFS/GEFS outliers, especially along and east of I-95. With that said, the 6/12z model suites should put the remaining puzzle pieces together as we will sit 60 hours from the event. Three scenarios remain: 1) The phase of all of these upper level features occurs too late, with low development offshore and too far southeast. 2) Similar to scenario 1, but an inverted trough on the back side of the low brings snow to eastern portions of the region. 3) Low develops closer to shore and strengthens along Delmarva Peninsula, resulting in the most snow for our areas. We still remain in the "wait and see" period with this storm, so just take precautions now, should the worst case scenario play out. Having a preparedness kit stocked up is never a bad idea. Lastly, no matter what plays out further east, it seems much more likely that the mountains will see another round of upslope snow. The key here will be with how much we manage to get. That will depend on the storm track as well, but it seems much more likely for at least advisory to perhaps warning level accumulations through the Sunday/Monday period along/west of the Allegheny Front. Of note, the higher sun angle later in the season becomes a factor especially for lower elevations if precip occurs during the day. Cold air will also be filtering into the region as the storm in question approaches, rather than being locked-in beforehand, which offers another layer of uncertainty in p-type.

I'd be surprised if any watches get issued before the 00z runs. Maybe after the 12z runs tomorrow?

I have my doubts about the totals at DCA. Currently, the lowest forecast temperature is 33 degrees at 7 am Monday. If the forecast temperature goes lower, I'll reconsider.

The 344 LWX snip... Unfortunately, the 12z model suites did nothing to help try to nail down a particular solution. On one end with the GEFS ensembles, we have seen an upward trend snow probabilities. However, in the latest EPS, the probabilities have dropped for portions of the area. Looking just at 3" snow probs in this timeframe, EPS dropped around 5-10% on average, while the GEFS jumped up significantly. The 12z Canadian ensembles also trended down, and by a good bit more than the EPS. I think this may ultimately translate to a net-zero change when it comes to the afternoon run of the NBM, but it will take some time to get that information, which may be elaborated more in the evening discussion. As described above, there are a substantial number of very complex variables at play here, and all of them have to come together at just the right time in just the right place in order for us to get snow, and especially significant snow. At present, there are three scenarios at play here. 1) The phase of all of these upper level features occurs too late, with low development offshore and too far southeast. 2) Similar to scenario 1, but an inverted trough on the back side of the low brings snow to eastern portions of the region. 3) Low develops closer to shore and strengthens along Delmarva Peninsula, resulting in the most snow for our areas. Exactly how all this occurs will be the difference between a couple of strung out waves passing through with little fanfare, a modestly impactful period of snow showers, and a more significant wintry precip event. We are firmly in the "wait and see" period with this storm, so just take precautions now, should the worst case scenario play out. Having a preparedness kit stocked up is never a bad idea. Lastly, no matter what plays out further east, it seems much more likely that the mountains will see another round of upslope snow. The key here will be with how much we manage to get. That will depend on the storm track as well, but it seems much more likely for at least advisory level accumulations through the Sunday/Monday period along/west of the Allegheny Front. Of note, the higher sun angle later in the season becomes a factor especially for lower elevations if precip occurs during the day. Cold air will also filtering into the region as the storm in question approaches, rather than being locked-in beforehand, which offers another layer of uncertainty in p-type.

LWX still up in the air at 1029... (snip) KEY MESSAGE 2...Low pressure tracking nearby brings a slight winter storm threat to the forecast area this weekend, followed by colder than normal temperatures early next week. A high energy but low predictability pattern looms for late this weekend. The pattern is forecast to be very amplified but in flux as a deep upper-level low digs out of the Gulf of Alaska inducing a downstream ridge-building event over the western U.S. This pattern amplification in the vicinity of western North America comes with low predictability due to data sparse regions over the North Pacific. Meanwhile, a pair of downstream upper lows interact/merge over Maritime Canada, with inherent low predictability with two cutoff lows interacting. If the upstream ridge-building event takes place a bit further west near 120 W longitude on Saturday, downstream trough amplification would be favorable for phasing over the mid MS Valley into the OH/TN Valleys Saturday night. A subsequent low track hugging the coast would become more likely in this scenario, aided by downstream blocking and a potential source of cold air given the upper lows over Maritime Canada and surface high pressure attempting to ridge into northern New England. There is an appearance of an inverted trough signature in a subset of model guidance over the last couple cycles, given interaction between a more amplified northern stream wave and a bit more distant offshore southern stream low. This could factor into precipitation amounts and placement. The 00Z ensemble suites (European, GFS, Canadian, Euro AIFS) all now have at least some snow across the area Sunday into Sunday night. This continues the trend toward higher QPF/snow amounts in the last 3-4 model cycles. Still, the spread remains quite large with many ensemble members indicating little to no snow, while others do show significantly higher amounts. There are several variables at play here that likely won`t be resolved with much more clarity for another day or two. This will determine the difference between a couple of strung out waves passing through with little fanfare, a modestly impactful period of snow showers, and a more significant wintry precip event. Of note, the higher sun angle later in the season becomes a factor especially for lower elevations if precip occurs during the day. Cold air will also filtering into the region as the storm in question approaches, rather than being locked-in beforehand, which offers another layer of uncertainty in p-type. Regardless of exactly how the details of the Sunday-Monday system play out, another shot of below normal temperatures is favored for early next week. Currently, high temperatures in the 40s (30s in the mountains) with lows in the 20s (teens in the mountains) are forecast through Tuesday night, with some midweek moderation after. (snip)

In my opinion, this is another reason to NOT get involved with social media.

Foggy in Arlington. Easily under a mile, maybe a bit over half.

Snip from LWX's latest AFD... KEY MESSAGE 2... Low pressure tracking nearby brings a slight winter storm threat to the forecast area this weekend. Further cold advection will take place Sunday as a trough dives across the Great Lakes. High temperatures on Sunday will be in the low to mid 40s with overnight lows dropping near or below freezing. An area of low pressure is forecast to develop over the southeast Sunday before deepening and tracking offshore towards New England Sunday night into Monday morning. This low pressure system brings increasing precipitation chances and a slight winter storm threat to the forecast area on Sunday. Models continue to show a wide range of solutions with uncertainty dependent on the strength, track, and timing of the low pressure system. Precipitation will likely begin as rain Saturday night into Sunday morning with snow mixing in overnight. With marginal temperatures throughout the day, the current forecast has a rain/snow mix likely throughout the day with precipitation type becoming primarily snow as temperatures fall Sunday night. Confidence in accumulating snow remains highest in the Alleghenies where temperatures are more favorable, although global guidance continues to show some degree of accumulating snow further east. There has been some increase in the probability of minor impacts via WPC`s probabilistic WSSI. With this event being 4 days out, there remains plenty of time for any variable to change and thus greatly altering the forecast. If this threat materializes, travel disruptions are possible but we will continue monitoring as this event gets closer.

Please NO MORE I**!!! Its cousin snowcrete is (still) bad enough. Spots that used to be grass are starting to reappear, though. (wish those piles would finish disappearing...)

From the latest LWX AFD... KEY MESSAGE 2... Colder this weekend with potential for coastal storm, with colder temperatures persisting into early next week. Pattern supportive of leader-follow system this weekend with leader coming Friday and follower Sunday. Upper-level trends over the past few synoptic runs in deterministic and ensemble guidance have trended slightly more favorable for a Sunday system to materialize. More blocking in the Atlantic with a ridge going up offshore has trended to more amplification and models hinting at a storm (potentially bigger). However, just as many ensembles show a weaker system with ridging and suppressed storm out to sea. Moisture does appear to be plentiful; the main uncertainty is how much cold air there will be to work with. Would need a stronger system to get impactful snow outside terrain in order to get more cold air brought into system. Even if a storm does develop, may not be all snow outside mountains with rain mixing in. Overall H5 pattern does support upslope snow behind the departing trough. To show the uncertainty, the EPS 50th percentile is currently a coating to an inch for most (several inches in the Alleghenies), while the 90th percentile is a solid 10-15". If the storm does come to fruition, it could be rather impactful as seen in WPC WSSI-P probs. Could also be a swing and a miss; time will tell. Regardless of this precipitation uncertainty, model guidance is in general agreement for a period of colder temperatures following this system, and the start of next week could trend significantly colder with highs in the 20s for most of the region.

Just about all the DC-area schools announced closings tomorrow. ((Maybe realizing that it kids stand on ice they can't break ice, nor homebound either?))

Just about all the DC-area public schools have announced they will close Tuesday, except DC itself and St. Mary's haven't declared yet.

looked out my window in Arlington VA and saw a millimeter or two of ice on top of tree twigs.

Outside my window in Arlington VA, ice pellets are starting to stick to the topside of branches. This may suggest they were partially melted, and refroze on contact. Something is STILL falling, and I think it all got to be a bit much for those trying to clear the path from their front door for the first time today. Visibility is better, but still not great. I just checked the NWS website that is reporting 1.25 mile visibility at DCA.

About the only "DC-area" public schools NOT on WJLA's closure list for tomorrow right now are Calvert, St. Mary's, Manassas Park and Spotsylvania. ((The list even included Berkeley and Morgan closures thru Tuesday.))

Visibility in Arlington back under a half-mile. Some very tiny precipitation is falling. Maybe an inch of sleet stuck to cleared sidewalks??

Visibility back to about 1/4 mile in Arlington VA. Looks like sleet (bouncing off tree twigs).

More sleet in Arlington VA. The sidewalk where the first neighbor shoveled out is coated. Visibility back down to 1/2 mile.

A kid (parent in tow) just sledded down part of my Hill of Doom. A couple more neighbors started to clear paths from their front doors.

One of my neighbors in Arlington VA came out immediately after the changeover and started shoveling. Some snow DID get scraped off my Hill of Doom, BUT with the sleet, and freezing rain potential, the street might get up to its old tricks again.

Eyeballs estimated about 4-4.5 inches in Arlington VA before the sleet? arrived a bit before 8 am. Didn't see-hear any pingers, though- maybe the teeny specks are rimed?

Been awake since 2 am (thank you tea). Fox weather was doing special broadcasting at 4 am. DC TV channels 4, 5 and 9 kicked in at 6 am with coverage, including their closures lists. Channel 4 estimated another 6 inches for the DC metro, while channel 5 thought there would be 3-5 inches added. Almost time to see what channel 7 will say.

Eyeballing out my window in Arlington, looks like 2 1/2 inches on fencetops. Just MAYBE my Hill of Doom has been plowed, but hard to tell. Visibility down to 1/2 mile, or maybe a bit less.

And the 815 ice forecast called for 0.16" in DC.

LWX's 816 am forecast now calls for 8 inches expected in Washington.