Monday, February 18, 2019


into Wed AM, then changing to sleet & freezing rain by Wed PM.

Below is our latest anaylsis and revised scenarios reposted from our central Maryland facebook update on Sunday evening. Check back during the day for additional information and updates.

1. STERLING VA NWS SNOW MAP as of 9:54 AM MON 2/18

  • Approximately 1.0" of liquid or more is expected across the region from Tuesday night to Wednesday night.  Some will be lost to evaporation in the first few hours of Tue evening, and another amount to sleet or freezing rain later on Wednesday. 
  • It is reasonable to expect at least 0.75" is convertible to snow for areas under a Winter Storm Watch. 
  • With temperatures around 30-32 F for most of the region during the event, a general 10:1 snow-to-liquid ratio can be expected until there is a change to sleet/freezing rain.

  • The second image is NOAA's precipitation projection from Wednesday morning through Thursday morning. Excessive rain is lurking just to the south of the Mid-Atlantic with amounts of 2" and higher for just a 24 hour period.
  • How to know if the forecast is on track? It's all about position of the High. We are closely watching for movement of the surface High in New England. 
  • If the center ends up moving back over SE Canada, the Mid-Atlantic may turn colder and receive higher snow amounts for longer. 
  • If the High shifts further east, expect a mix to move in sooner and cut down on snow totals.

4. STORM SCENARIOS as of 11:00 AM MON 2/18
  • Slight revisions to the scenarios posted Sunday morning are shown below, accounting for potential movement of the High as noted in item #2.
  • By this evening, as we approach issuance of any Winter Storm Warnings, our team will publish the Regional Snow Map in accordance with our "24 hours out" rule on snow projections. 
SCENARIO A (60% chance): Another Snow-To-Ice Mess
This projection produces an all-snow start across the region late Tuesday into early Wednesday, with accumulation prior to the Wed AM commute.
  • A coating to 2" from Bowie, MD / eastern DC metro to Anne Arundel County and down to southern MD before a wintry mix works in by mid-morning.
  • 4-6" for all other areas from there north and west to the 95 corridor and the I-70 corridor. Snow remains the primary precipitation typle through Noon Wednesday, with sleet mixing by the afternoon hours. Amounts exceeding 6" are possible in the higher elevations along western portions of the 70 corridor and near the PA line.
  • Sleet & freezing rain would persist into early Thursday, changing to all rain area wide soon after the AM commute, but not soon enough to prevent significant ice problems on roadways.

SCENARIO B (20% chance): A 2-Day Big Kahuna Blowout

This projection takes into account the trend we have seen this winter, of storms "overperforming" with higher accumulations than forecasted for some areas, or warnings and advisories being extended due to a longer duration than first expected. 

  • 6-10" for areas along and north of I-70 into southern PA, with greater amounts along the PA/MD line
  • 4-6" for areas south of I-70 to the 95 corridor, with mixing to develop by mid-afternoon Wed after bulk of heaviest snow has fallen. 
  • 2-4" for areas south and east of 95 to the Route 50 corridor, including DC metro, with mixing to develop by 12 PM.
  • Coating - 2" from below Route 50 to southern Anne Arundel Co & Southern MD, with mixing by mid-morning. 

    SCENARIO C (20% chance) for a Cold, Curmudgeony Rain.

    Snow area wide by the Wed AM commute turns to a mix along the 95 corridor by mid morning, then to rain, with all other areas turning to a rain/sleet mix by Noon.

    • Due to copious warm air transport from the Atlantic, where water temperatures off the VA coast are in the low 50s, snow totals are held down to generally 2-3" for areas south of I-70, with up to 1" in the DC & Baltimore metros.

    • Warm air at the mid-levels, combined with a southeast surface wind, erodes boundary layer cold air and mixing develops soon after sunrise. All turns to rain by Noon Wednesday into the evening, becoming heavy at times into Thursday.

    Additional information, weather decision graphics and our long-awaited regional snow map will be added later today, so please check back by lunchtime and this evening.

    Winter Stormcast Team collaborators:
    Forecasters Jason M., Ira W., Keith K., J. McDuffie, R. Foot

    Sunday, February 17, 2019

    1 comment:
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    The FIRST day of Summer for some,
    might also be their LAST day of the school year.

    At the rate this winter is going, unorthodox approaches may have to be 
    considered by school districts caught in the calendar conundrum.

    LAST UPDATED 3:30 PM SUN 2/17/19
    • HIGH PROBABILITY FOR A SIGNIFICANT WINTER STORM TO IMPACT A MAJORITY OF THE EASTERN MID-ATLANTIC early Wednesday into Thursday from Southern PA to Western MD to Northern & Central VA to the Baltimore-Washington metro area. The Sterling VA NWS has for the first time this season outlined an enhanced threat for a widespread impactful event as shown below. (NWS)
    • LIQUID EQUIVALENT EXCEEDS 1.0" for many areas for the Wed-Thu postion of this event, with overnight start and sub 32 F temperatures indicating potentially high amounts of snow, sleet & freezing rain may occur. (NOAA WPC)
    • HOW MUCH SNOW & ICE? Northern areas which remain cold enough for mostly snow could exceed 5" with areas closer to the Bay turning to a wintry mix Wednesday morning, reducing snowfall totals east of the I-95 corridor but increasing the sleet & freezing rain threat to 0.25" of ice in those areas (US GFS model)

    But, significance of this storm 
    goes beyond just weather:

    Could this system be the season's first "Big Kahuna?" 
    • BELOW IS THE NOAA DAY 4 PROBABILITY MAP depicting the chance of frozen precipitation occuring in the shaded areas would exceed 0.25" of liquid equivalent. In the acacdemic study of statistics and probability, this high level 4 days in advance reflects confidence in the atmospheric dynamics required to produce this type of outcome.
    • WHY IS THIS IMPORTANT? The extensive coverage of green shading, which represents a moderate 30-50% chance of the 0.25" liquid equivalent threshold, is the first time in the 2018-19 season such a strong probability has been posted for such a large portion of the densely populated Mid-Atlantic region. 
    • WHAT IS A "BIG KAHUNA"? First coined by FF writers and readers on this site in 2005, this term is assigned by our team to denote a widespread moderate to high impact winter storm affecting a large portion of the Mid-Atlantic (or other region), and produces 6-12" of snow. However, at 4 days out, uncertainty precludes us from determining this early it WILL be a Big Kahuna.

    Preliminary timing & scenarios:

    Columbia, Howard County, MD is chosen as the representative location to illustrate location of the dividing line on Wednesday between two areas of different outcomes:
    • Areas to north & west of Columbia/I-95 that may remain mostly snow for a majority of Wednesday, and;
    • Areas south & east of Columbia/I-95 which are more likely to transition to a wintry mix of sleet, freezing rain and rain.

    SCENARIO A: Another sloppy schedule mess
    • Despite probable Winter Storm Warnings posted throughout the region by the time Wednesday morning arrives, dry air and a slower onset of precipitation results in some schools & colleges deciding to open on time, observing that no snow is occuring at 5 AM.
    • Soon after students arrive, heavy snow sweeps east to the 95 corridor by 8 AM. Another round of hastily announced early dismissals occur. This time, condition decline rapidly as snowfall rates quickly hit 1.0" an hour. The result is thousands of commuters and parents take hours to get home, harkening back to an era when less detailed technology made these kinds of situations commonplace. 
    • For areas north and west of I-95, north of I-70 into northern VA, central MD and the I-81 corridor into southern PA, snow continues unabated through 6 PM, with many areas exceeding 6", with higher amounts along the PA line. For areas south & east of 95, including much of the DC metro, mixing with sleet and freezing rain develops by mid morning, turning the PM commute into a complete icy lockout disaster. By Thursday morning, overnight runoff and refreezing has created another ice nightmore, although all precip is turning to rain around sunrise.
    • Below: European model projected precipitation type by 7 AM Wednesday:

    • Light to moderate snow develops in the early morning hours of Wednesday, with at least 1.0" on the ground in all locations north of the I-66 corridor in Washington, DC. Many schools and colleges elect to close outright instead of a 2-hour delay with re-evaluation, as it is clear conditions will worsen through the day.
    • Colder than expected surface and upper level temperatures permit snow to persist longer than forecasted, and over a larger area. A change to sleet & freezing rain is delayed until late afternoon for areas east of 95, and never occurs for areas north of I-70 to the PA line & southern PA. Winter Storm Warnings are extended into early Thursday for heavy snow in the north, and up to 0.30" ice along from I-70 south to I-95 and east to I-97.
    • By Wednesday evening, snowfall totals approach 12" for areas north of I-70 and along I-81. In northern Maryland, some parts of the "the North 4" of Frederick, Carroll, Baltimore & Harford counties exceed 12" in areas bordering the PA line, and it is still snowing by nightfall. Cecil County is vying for recognition with a respectable 8-10" of snow. Areas from I-70 to 95 are seeing a sleet/freezing rain mix producing a glaze on top of 4-6" of snow, and from DC to southern Maryland precip is turning to rain by Noon after 3-4" of snow. 
    • Below: NOAA Global Forecast System (GFS) model for 7 AM Wednesday.

    And finally, the weather map feature 
    that will determine what really happens:
    Position of the High pressure in Canada.
    • An old forecasting rule proven correct time and again is simply this: Predict the High and you predict the storm. If the High in SE Canada positions where shown by Wednesday morning, Scenario B has a high chance of succeeding. 
    • If the High in upstate NY/SE Canada pulls east earlier than expected, the other Low pressure features will convert this setup into a snow-to ice-to rain outcome similar to what has occured twice in the past 2 weeks. 

    And now, let the speculation games begin!

    Which scenario are you siding with? 
    Are you prepared for the consequences of that? 
    We shall see and time will tell.