Area Forecast Discussion
Issued by NWS Sterling, VA

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000 FXUS61 KLWX 200743 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 343 AM EDT Fri Oct 20 2017 .SYNOPSIS... High pressure will remain across the Mid Atlantic through this weekend. A low pressure system will affect the region early next week. && .NEAR TERM /THROUGH TONIGHT/... A very weak cold front currently pushing south across Pennsylvania will cross the region early this morning. This front is keeping the wind a bit more stirred up, so temps have not dropped as cool as recent mornings, and there is a bit less fog. The front should result in little if any cooling today, with its main effects being a wind shift and a bit more gustiness over the waters. Otherwise, it should remain mostly clear with temperatures returning to the 70s in most spots. Tonight, high pressure builds overhead once again, so we should have a decent inversion and a bit more potential for patchy fog. Otherwise, mostly clear and cool with lows mostly in the 40s. && .SHORT TERM /SATURDAY THROUGH SUNDAY NIGHT/... High pressure will slowly shift east off the coast Saturday and remain there Sunday. Winds will gradually become southerly and temps aloft will warm a bit, but otherwise, mostly sunny skies and light winds should prevail, as we generally remain under the light gradient close to the center of the high. High clouds will likely increase Sunday, and by Sunday night, the next system may bring enough moisture and lift to spark a shower in the mountains late at night, but that`s about it though the short term. Highs will be in the 70s, with Sunday probably just slightly warmer than Saturday, and lows in the 40s Saturday night, rising to the 50s Sunday night as the clouds and southerly flow ahead of the next front keep it mild and better mixed. Patchy fog will be a concern late at night and early in the morning hours, probably more so Saturday night/Sunday morning than Sunday night. && .LONG TERM /MONDAY THROUGH THURSDAY/... Main story of the long term period is a powerful upper trough/front. Phasing between northern and southern streams seems imminent given amplification of flow upstream and blocking downstream. The juxtaposition of the right-entrance region of the upper-jet, strong mid-level vorticity advection and frontogenesis in the low levels would suggest high probability for widespread and potentially heavy precipitation. Given antecedent drier-than- normal conditions and progressive (quick moving) nature of the system, widespread flooding seems unlikely. The rain should instead prove beneficial. Ensemble mean QPF is around 1-1.25", mainly focused in 00z Tuesday through 12z Wednesday. The tight gradient between deep troughing to the west and ridging to the east is expected to result in a stribg southerly low-level jet (marked by 50 knot 925 mb flow). This may result in gusty winds reaching the surface, particularly with any heavier showers or embedded convective elements. Timing and strength differences are still evident amongst the latest deterministic/ensemble guidance, but gusty winds and heavy downpours seem most likely sometime in the Tuesday morning through Tuesday evening timeframe. Cooler temperatures and the potential for showers are expected during the middle of next week in the wake of the surface front as upper troughing stalls overhead. Some ensembles hint at coastal low development late next week, but historically it becomes difficult to see substantial impacts from such a system once the surface front has already rolled through. && .AVIATION /08Z FRIDAY THROUGH TUESDAY/... Patchy fog possible at MRB/CHO/IAD over the next few mornings, but odds are low and should generally be MVFR at worst. Otherwise, VFR through Sunday night all terminals with light winds under high pressure. Sub-VFR possible Mon AM (fog/low CIGS), then again late Mon night with lower CIGS/developing showers ahead of an approaching cold front. LLWS possible by Tuesday with strong flow (40-45 kts) around 2kft. Models differ on timing but agree on overall setup. Widespread restrictions likely Tuesday with showers (some heavy) expected. Gusty surface winds possible. && .MARINE... A weak passing cold front early this morning will stir the winds up over the waters. Soundings show some sustained winds of 20-30 knots just above the surface, and while upstream areas near shore will probably have a surface inversion, just off shore, especially as one heads further downstream, that inversion should be weaker if present at all given the warm water temps, so expect some of that wind to mix down. As temps warm and the winds just above the surface diminish during the day Friday, gusts should drop below SCA criteria. After midday, winds should generally stay below SCA criteria under high pressure through Sunday night. There is a small risk of southerly channeling Sunday night as the next front approaches, but right now think odds are pretty low as low level jet stays pretty weak. SCA likely Monday as high pressure departs and southerly flow develops. High end SCA seems likely Tuesday with strong southerly LLJ. Gales possible as well, especially in heavier convective elements ahead of an approaching cold front. && .TIDES/COASTAL FLOODING...
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Tidal anomalies will increase early next week as southerly flow develops. Chances for minor tidal flooding increase as southerly flow strengthens ahead of an approaching cold front Monday night into Tuesday.
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&& .LWX WATCHES/WARNINGS/ADVISORIES... DC...None. MD...None. VA...None. WV...None. MARINE...Small Craft Advisory until noon EDT today for ANZ531>534-537- 539>541-543. && $$ SYNOPSIS...RCM NEAR TERM...RCM SHORT TERM...RCM LONG TERM...DFH AVIATION...RCM/DFH MARINE...RCM/DFH TIDES/COASTAL FLOODING...DFH

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