Area Forecast Discussion
Issued by NWS Sterling, VA

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000 FXUS61 KLWX 190155 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 955 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/... High pressure will push east of the region overnight. Warm advection/isentropic lift will start to bring increasing clouds overnight, and some light rain/drizzle will be possible across portions of central VA by dawn. Lows will remain a little cool for mid March, but not too far below normal. && .SHORT TERM /6 AM MONDAY MORNING THROUGH WEDNESDAY/... Guidance remains in general agreement regarding overall system progression, but the devil is in the details. Many models show temperatures for a significant depth of atmosphere very close to freezing through the core of the storm, particularly the portion on Tuesday, and that makes snowfall forecasting particularly difficult. In general, low pressure will advance east across the southern tier of the US on Monday, with warm advection/isentropic lift developing ahead of it to our southwest. Some may spread over our southwestern zones, so rain is possible through much of Monday over central VA, but further north, over the bulk of the CWA, rain chances do not increase significantly until the system starts turning north on Monday night. Latest temperature profiles suggest it will start as rain just about CWA wide Monday night, but that will not last long. As low pressure redevelops in the Carolinas early Tuesday, cold air will start being drawn southward, first in our northwestern zones, and then gradually spreading south and east through the rest of Tuesday and Tuesday night. The process will proceed faster Tuesday morning as the first low pressure pushes northeast off the coast, with rain changing to snow and sleet down to around the I-95 corridor by midday or early afternoon. After that, a relative lull in precipitation is expected before another low develops with a second upper level impulse, which will then push northeast off the Carolina coast on Wednesday. As this low moves northeast Tuesday night, it too will draw in the cold air southeastward, allowing those areas which have not yet changed over by Tuesday evening to finally switch to all or mostly snow by late morning Wednesday. This switch in p-types gives us headaches with the first low in particular, given we may have heavy precip falling as it is changing to snow and sleet, but for now hedged towards the warmer/less snowy solutions for this wave. This means that advisory snowfall is not likely along I-95 on Tuesday morning, at least not the standard criteria (the lower rush-hour criteria may still be met). Standard criteria is now looking more likely for the Tuesday night/Wednesday morning low pressure wave, so increased snowfall in particular during this period. A this wave finally pulls away later in the day, snowfall should gradually wind down. Temperatures will be near normal or just a little below on Monday night, but trend significantly below normal Tuesday and Wednesday. Lows Tuesday night will be just a bit below normal. This will all be due to the ongoing cold advection and combination of ongoing clouds and precip. Hedged highs several degrees below guidance blends. && .LONG TERM /WEDNESDAY NIGHT THROUGH SUNDAY/... Low pressure will be moving away Wednesday night. 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 /02Z MONDAY THROUGH FRIDAY/... VFR conditions through Monday afternoon for all terminals. Sub- VFR conditions will develop overnight Monday into early Tuesday with IFR conditions possible before daybreak Tuesday and continueing into Wednesday 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. VFR conditions expected on Thursday and Friday with high pressure in control. && .MARINE... No marine hazards through Monday. Small craft advisory conditions Monday night 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...Small Craft Advisory from midnight Monday night to 6 AM EDT Tuesday for ANZ531>533-537-539>543. Small Craft Advisory from 6 PM Monday to 6 AM EDT Tuesday for ANZ534. && $$ SYNOPSIS...KLW/RCM NEAR TERM...KLW/RCM SHORT TERM...KLW/RCM LONG TERM...IMR/RCM AVIATION...IMR/KLW/RCM MARINE...IMR/KLW/RCM is the U.S. government's official web portal to all federal, state and local government web resources and services.