Area Forecast Discussion
Issued by NWS Sterling, VA

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153 FXUS61 KLWX 181921 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 321 PM EDT Sun Mar 18 2018 .SYNOPSIS... High pressure will build overhead through tonight. Low pressure will pass through the Tennessee Valley on Monday, with a coastal low developing off the mid-Atlantic coast Tuesday. A third low pressure system will follow the coastal low and pass our region to the south Wednesday. High pressure will return for late next week. && .NEAR TERM /UNTIL 6 AM MONDAY MORNING/... Current temperatures are on track to reach our high forecasted temperatures for the day. Currently temperatures are widespread low to middle 50s. Dewpoint temperatures had to be brought down 5 to 7 degrees as there are widespread dewpoint temperatures in the upper teens to low 20s. High pressure will continue to build overhead through tonight and provide dry conditions to the region. Plenty of sunshine the rest of the afternoon. Seasonably chilly conditions with clear skies most of tonight before clouds increase at the mid-levels late. && .SHORT TERM /6 AM MONDAY MORNING THROUGH TUESDAY NIGHT/... High pressure, providing dry and seasonable conditions, will move from near the coast out to sea Monday to allow for a developing coastal low to begin its formation Monday night. Temperatures will start off near or slightly below freezing Monday morning as clouds continue to increase, as well as dewpoint temperatures. High temperatures Monday should reach 50 degrees or better once again across the region. Through the period of Monday night through Tuesday night, a low pressure system will reach the Appalachian Front and transfer its energy to the East Coast to form the coastal low that is expected to intensify as it moves to the northeast. Still, there are variations in the timing of the formation of this coastal low, the track of this low, and the precipitation type. The NAM, GFS and European model do not agree on a lot of these mentioned factors prior to 18z Tuesday. The NAM during the past couple model runs has been flipping back and forth with significant amounts of snow to a small amount of a mix of rain and snow after some rain. The GFS has been consistent with snow amounts across the region with an average of 2 to 4 inches of snow after a period of rain. The latest European model should be used with a little caution, considering it is showing most areas receiving several inches of snow Tuesday into Tuesday night. A couple of days ago, it was revealing the same scenario, but the last model run or two, it backed off on the amounts. Lately, and possibly to this minute, our forecast has been a blend of the GFS and European model with a lean toward the GFS model. Where the three models tend to agree is around the 18z time of Tuesday. All three models have the newly-formed coastal low on Tuesday about 50 miles to the east of Norfolk, Virginia with a trailing trough of low pressure to the southwest then northwest into southwest Virginia. At least with this position, we have a potential of seeing several inches of snow, especially north and west of the I-95 corridor. As the coastal low passes by to our east then northeast later Tuesday morning and Tuesday afternoon, colder air will get drawn in from the north. The colder air will move into place shortly ahead of the next shortwave that will be on the heels of the first coastal low. A lull in the precipitation is most likely later Tuesday in between the two lows. Precipitation will fill in as the second coastal low develops Tuesday night. Guidance still diverges on how strong the low will be and consequently how much precipitation fills in. It does appears that the best chance for precipitation will be across the west and south...closer to the track of the low. Thermal profiles will be colder due to northerly winds and this will cause the p-type to be snow for most areas. Accumulating snow is possible. && .LONG TERM /WEDNESDAY THROUGH SUNDAY/... A secondary low pressure system will be near our region on Wednesday morning and will transfer its energy off of the Mid- Atlantic coast. This system will intensify offshore as precipitation continues over our area for part of the day Wednesday before the system moves away later that day. Cold air aloft and near the surface supports a p-type of snow over the region with model ensemble average QPF amounts between a tenth to three tenths of a inch. A high pressure system builds over our region Thursday into Friday bringing dry conditions over the area before another low pressure system and its frontal boundary affect us for next weekend. && .AVIATION /19Z SUNDAY THROUGH FRIDAY/... VFR conditions through Monday evening for all terminals. MVFR conditions will develop overnight Monday into early Tuesday. There is a chance that IFR conditions could evolve as early as daybreak Tuesday and continue through Tuesday night depending on the coverage and intensity of the wintry mix or snow. Winds becoming northeast then southeast Monday, then back around to the northeast then north as the coastal low develops and moves up the coast. Sub-VFR conditions expected for some part of the day on Wednesday with a low pressure system near our region by during this time. Breezy conditions expected on Wednesday, some gusts up to 20 kts possible. VFR conditions expected on Thursday and Friday with high pressure in control. && .MARINE... No marine hazards through Monday. Small craft advisory conditions possible Monday as the coastal low starts to develop along the coast. Gale warnings possible Tuesday into Tuesday night, mainly for the central Chesapeake Bay. Afterwards, small craft advisories are possible as the gusty winds diminish. Wind gusts will be above small craft criteria on Wednesday as low pressure system moves near our region... therefore a small craft advisory is likely. Winds should remain below criteria on Thursday and Friday. && .LWX WATCHES/WARNINGS/ADVISORIES... DC...None. MD...None. VA...None. WV...None. MARINE...None. && $$ SYNOPSIS...KLW NEAR TERM...KLW SHORT TERM...KLW LONG TERM...IMR AVIATION...IMR/KLW MARINE...IMR/KLW is the U.S. government's official web portal to all federal, state and local government web resources and services.