Area Forecast Discussion
Issued by NWS Sterling, VA

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000 FXUS61 KLWX 211227 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 827 AM EDT Mon Aug 21 2017 .SYNOPSIS... High pressure will move offshore later today. A disturbance will bring showers and thunderstorms to parts of our region this afternoon and evening. High pressure will regain control overnight tonight through Tuesday, before a strong cold front pushes across our region and brings another threat for showers and thunderstorms Tuesday night into Wednesday. The front could stall to our southeast, while a second area of high pressure builds in from the west later this week. && .NEAR TERM /THROUGH TONIGHT/... We have our eyes on a shower/weak thunderstorm over the Bay. Light winds will become southerly at 5 to 10 mph. This will usher in additional low-level moisture to help fuel some showers and thunderstorms this afternoon and into this evening. As the high pressure overhead inches offshore today, a disturbance currently over northwest Ohio will make its way across and just north of the Mason-Dixon Line later this afternoon and this evening. This disturbance, combined with daytime heating, increased low-level moisture, indicated by dewpoint temperatures in the lower to middle 70s, and some wind shear should generate showers and thunderstorms this afternoon and evening. The main threats will be damaging winds, some hail, and very heavy rainfall due to precipitable water values between 1.80 and 2 inches. Most of this activity will be confined to an area from US 15 to the Chesapeake Bay and south to the Fredericksburg. Timing on this activity seems to be between 3 and 9 pm. There could be an isolated shower or thunderstorm develop over the mountains of western Maryland or northern Shenandoah Valley as early as noon. Also, some of this evening`s activity could linger longer into the night, perhaps close to midnight before weakening or dissipating. As rain activity lessen overnight, some patchy fog could develop in areas where it has rained and where dewpoint temperatures remain near 70 or in the lower 70s. Tonight`s lows will be in the lower to middle 70s. && .ECLIPSE... A few eclipse facts... The sun is about 390 times larger than the moon, and the sun is farther away from the earth than the moon coincidentally by a factor of 390. When these align just right the moon blocks the sun`s photosphere as viewed from earth, and we can see the corona. This is different each eclipse due to the shape of the magnetic field emanating from the sun. The moon is receding away from the earth at a rate of ~4 cm per year. Eventually total solar ecipses will be impossible. I`ve heard/read discussion about today`s being a "once in a lifetime" eclipse. Actually there will be another on April 8, 2024 across the eastern half of the US. It will stretch from San Antonio, TX to Caribou, ME. Cities that will experience totality include Dallas, Little Rock, Indianapolis, Cleveland, Buffalo, and Burlington. Some of these will experience close to 4 minutes of totality. In 2024 Washingon DC will experience 87% coverage (81% today), Baltimore 88% (today 79%). The jackpot city is Paducah, KY. The two eclipse tracks across the states form somewhat of an "x" with Paducah at the center of it. Today it will experience 2:24 of totality. In 2024 the sun will be covered for 1:45. Enjoy today`s event, observe safely, and see you in 2024. && .SHORT TERM /TUESDAY THROUGH WEDNESDAY NIGHT/... A lull in the shower and thunderstorm activity could fulfill most of Tuesday. High pressure will regain control and allow for dry conditions. There is a chance for a shower or thunderstorm along the Mason-Dixon region and northeast Maryland Tuesday afternoon and evening as a weaker disturbance moves east across Pennsylvania. Otherwise, most other places will be dry. Highs will reach the middle 90s with dewpoint temperatures in the lower to middle 70s once again. The timing on a strong cold front remains in question late Tuesday into Tuesday night. We have a chance for showers and thunderstorms across the northern and western half of our region Tuesday night for now. Tuesday night`s lows will only bottom out in the middle 70s. The chance for a couple of showers linger Wednesday in the west, central and southern areas. This is heavily weighted on when the cold front can pass through the region. High pressure should gradually build in from the west Wednesday night and bring cooler and more comfortable conditions to the region. Highs Wednesday will be middle 80s, cooler than previous days. Lows at night into the middle to upper 60s. && .LONG TERM /THURSDAY THROUGH SUNDAY/... The extended forecast is blissfully quiet as high pressure builds northeast from Canada. Not only will the chance for precip be nil, but temperatures will once again be atypically mild. The only cause for concern will come from the upper level trough axis dropping across the northeastern US. This feature may result in added cloud cover. We shall see if subtleties from associated weak vorticity and jet energy lead to a swing toward scattered showers in future model runs. && .AVIATION /13Z MONDAY THROUGH FRIDAY/... VFR conditions through Wednesday night. Any heavy showers or thunderstorms Monday afternoon and evening, as well as late Tuesday and Tuesday night could reduce conditions to MVFR or IFR briefly. Winds light and variable becoming south at 5 to 10 knots today. Winds becoming southwest around 10 knots tonight through Tuesday, then backing to the northwest at 10 knots Tuesday night and Wednesday depending on frontal passage. VFR conditions anticipated Thu-Fri under building high pressure. && .MARINE... No marine hazards expected through Tuesday morning. Have opted to issue Small Craft Advisory for Tuesday afternoon-evening at this time; gusts to 20-25 kt anticipated in mixing ahead of cold front. Behind the cold front at the end of the week, expect north winds. At this point, speeds look to be less than 15 kt. && .LWX WATCHES/WARNINGS/ADVISORIES... DC...None. MD...None. VA...None. WV...None. MARINE...Small Craft Advisory from noon to 11 PM EDT Tuesday for ANZ530>543. && $$ .UPDATE...WOODY! .PREVIOUS...KLW/HTS

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