Area Forecast Discussion
Issued by NWS Sterling, VA

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000
FXUS61 KLWX 211858
AFDLWX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE BALTIMORE MD/WASHINGTON DC
258 PM EDT SUN SEP 21 2014

.SYNOPSIS...
LOW PRESSURE OFF THE THE MID-ATLANTIC COAST WILL CONTINUE TO MOVE
NORTHEAST. A COLD FRONT WILL CROSS THE REGION THIS EVENING INTO
THE OVERNIGHT. HIGH PRESSURE WILL BUILD OVER THE AREA MONDAY INTO
TUESDAY...THEN SHIFT NORTHEAST THROUGH THE LATE WEEK.

&&

.NEAR TERM /THROUGH TONIGHT/...
AS OF 18Z...COASTAL LOW IS CENTERED OFF THE NC/VA BORDER PER
REGIONAL IR/WV SATELLITE IMAGERY. A COLD FRONT IS OVER OHIO AND
SHIFTING EAST (THERE ARE SEVERAL WIND REPORTS OF 30 TO 35 MPH IN
IN/OH). A PREFRONTAL LINE OF SHOWERS IS ENTERING THE POTOMAC
HIGHLANDS. THIS IS THE SECOND WAVE OF THE DAY...WITH ANOTHER WAVE
OR TWO EXPECTED WITH THE FRONT ITSELF. THE INVERSION AT 700MB FROM
THE 12Z KIAD RAOB IS ONLY SLOWLY BEING ERODED PER CLEAR SKIES FROM
THE BLUE RIDGE TO INTERSTATE 95.

MLCAPE 500 J/KG CURRENTLY OVER THE SHENANDOAH VALLEY AND WEST PER
SPC ANALYSIS. MID AFTERNOON THUNDER THREAT IS ISOLATED AT
BEST (AND WEST OF THE BLUE RIDGE)...WITH MORE SCATTERED COVERAGE
WITH THE FRONT THIS EVENING WHEN MLCAPE SHOULD EXCEED 1000 J/KG.
ISOLATED THUNDER WORDING EXTENDS EAST TO THE INTERSTATE 95
CORRIDOR FROM 5 TO 8PM. SHOULD THE CAP BE COMPLETELY BROKEN...AN
85/62 INITIAL PARCEL WOULD ALLOW FOR AROUND 1000 J/KG SBCAPE.

IN SPITE OF FROPA TIMING AFTER SUNSET EAST OF I-95...PRESENCE OF
INSTABILITY SHOULD ALLOW ISOLATED THUNDER.

NWLY FLOW SPREADS IN BEHIND THE FRONT WITH GUSTS GENERALLY 25 TO
30 MPH.

MIN TEMPS ELEVATED IN MIXING FLOW IN SPITE OF COOLER AIR COMING
IN. MID TO UPR 50S INLAND...LOW TO MID 60S NEARSHORE. UPSLOPE RAIN
SHOWERS CONTINUE THROUGH THE NIGHT WEST FROM THE ALLEGHENY FRONT.

&&

.SHORT TERM /MONDAY THROUGH TUESDAY NIGHT/...
A FEW UPSLOPE SPRINKLES WEST FROM THE ALLEGHENY FRONT MONDAY
MORNING. OTHERWISE DRY WITH NWLY GUSTS 25 MPH DECREASING THROUGH
THE AFTERNOON. GOOD MIXING AND 850MB TEMPS OF 4-6C SUGGEST MAX
TEMPS IN THE LOW TO MID 70S.

HIGH PRESSURE CONTINUES TO BUILD MONDAY NIGHT WITH THE GRADIENT
SLACKING PARTICULARLY WEST OF THE BLUE RIDGE BY DAYBREAK TUESDAY.
WITH DRY AIR IN PLACE...MIN TEMPS COULD BE IN THE MID 30S IN
WRN VALLEYS AND THE POTOMAC HIGHLANDS. SINCE THIS IS THE SHELTER
TEMPERATURE (ABOUT SIX FEET OFF THE GROUND) GROUND TEMPERATURES
COULD HIT FREEZING AND THUS CAUSE FROST. A MENTION FOR FROST WILL
BE PUT IN THE HWO FOR OUT THERE. OTHERWISE...SUB NORMAL MIN TEMPS
WITH LOW TO MID 40S PIEDMONT AND WEST...UPR 40S INLAND COASTAL LOW
LANDS AND LOW TO MID 50S URBAN/NEARSHORE.

HIGH PRESSURE SHIFTS TO THE NORTHERN MID-ATLANTIC TUESDAY AND
TUESDAY NIGHT. CONTINUED DRY CONDITIONS WITH NLY FLOW AND BELOW
NORMAL TEMPS.

&&

.LONG TERM /WEDNESDAY THROUGH SUNDAY/...
HIGH PRESSURE OVER NEW ENGLAND WILL SLOWLY MOVE OUT INTO THE
NORTHWESTERN ATLANTIC AS A BROAD AREA OF LOW PRESSURE DRIFTS
WESTWARD INTO THE SOUTHEASTERN CONUS. A MOIST EASTERLY FLOW IN A
TIGHTENING GRADIENT BETWEEN THESE TWO FEATURES WILL RESULT IN
INCREASING CLOUDS THROUGHOUT THE DAY WEDNESDAY...WITH ABUNDANT CLOUD
COVER IN A MARITIME AIRMASS EXPECTED TO PERSIST INTO THE END OF THE
WEEK.

THE OPERATIONAL 00Z ECMWF MODEL WAS DEFINITELY THE MOST AGGRESSIVE
WITH THE NORTHWARD EXTENT OF PRECIP ASSOCIATED WITH LOW PRESSURE TO
THE SOUTH IN THE THURSDAY-FRIDAY TIMEFRAME...BUT DOES NOT HAVE
OVERWHELMING SUPPORT FROM ITS OWN ENSEMBLE MEMBERS. THERE IS A HIGH
CHANCE FOR PERIODS OF DRIZZLE DURING THIS PERIOD GIVEN THE
PERSISTENT EASTERLY FLOW...BUT MEASURABLE PRECIPITATION ASSOCIATED
WITH THE LOW WELL TO THE SOUTH APPEARS RELATIVELY LOW AT THE MOMENT.
HAVE KEPT POPS VERY LOW FOR THIS REASON.

AS THE LOW PRESSURE OVER THE SOUTHEAST WEAKENS AND THE HIGH PRESSURE
MOVES FURTHER OFFSHORE...UPPER-LEVEL RIDGING WILL RE-STRENGTHEN AND
RESULT IN SOUTHERLY FLOW AT THE SURFACE WITH A WARMING TREND
EXPECTED BY NEXT WEEKEND.

&&

.AVIATION /18Z SUNDAY THROUGH FRIDAY/...
SWLY FLOW AHEAD OF A COLD FRONT. SCATTERED SHOWERS WITH ISOLATED
THUNDERSTORMS SPREAD EAST ACROSS THE DC METROS LATE THIS
AFTERNOON/EARLY EVENING. HIGHEST THUNDER THREAT IS KMRB WHERE A
VCTS 20Z TO 00Z IS IN THE TAF.

NWLY FLOW GUSTS 20 TO 25 KT BEHIND THE COLD FRONT TONIGHT THROUGH
MONDAY. VFR AND NLY FLOW THEN TUESDAY WITH HIGH PRESSURE SPREADING
NORTH OF THE AREA.

SUB-VFR CONDITIONS APPEAR INCREASINGLY LIKELY WITH LOW CLOUDS/FOG
MAKING A VISIT TO THE AREA WED-THU NIGHT. WINDS GENERALLY EASTERLY
10-15 KNOTS WITH A FEW GUSTS TO 20 KNOTS POSSIBLE.

&&

.MARINE...
LOW PRESSURE MOVES NORTHEAST FROM OFF THE DELMARVA. NELY FLOW
WILL BECOME SWLY AHEAD OF A COLD FRONT THAT WILL CROSS TONIGHT.
NWLY FLOW GUSTS 20 TO 25 KT STARTING AROUND MIDNIGHT...SO THAT IS
WHEN THE SCA BEGINS FOR ALL WATERS. SCA THROUGH MONDAY WITH GUSTS
IN THE MORNING AROUND 25 KT...SLOWLY EASING THROUGH THE
AFTERNOON.

NLY THEN NELY FLOW AS HIGH PRESSURE SHIFTS NORTH TUESDAY.

WINDS INCREASE TO NEAR SCA LEVELS DURING THE MIDDLE PART OF THE WEEK
IN A TIGHTENING PRESSURE GRADIENT BETWEEN A DEPARTING HIGH TO THE
NORTH AND LOW PRESSURE DRIFTING INTO THE SOUTHEAST CONUS FROM THE
ATLANTIC. WINDS/WAVES SUBSIDE AS THE TWO PRESSURE SYSTEMS WEAKEN AND
MOVE AWAY BY NEXT WEEKEND.

&&

.TIDES/COASTAL FLOODING...
NLY FLOW HAS LOWERED THE DEPARTURE BELOW HALF A FOOT. FLOW IS
COMING SWLY BEFORE A COLD FRONT THAT WILL CROSS THIS EVENING.
HOWEVER...THAT IS NOT FAVORABLE FOR COASTAL FLOODING. NWLY FLOW
WILL RETURN LEVELS NEAR NORMAL MONDAY.

TIDAL ANOMALIES WILL INCREASE IN PERSISTENT MODERATE EASTERLY FLOW
WEDNESDAY NIGHT-FRIDAY WHICH ALONG WITH A NEW MOON MAY CAUSE
COASTAL FLOODING AROUND TIMES OF HIGH TIDE STARTING LATE
WEDNESDAY.

&&

.LWX WATCHES/WARNINGS/ADVISORIES...
DC...NONE.
MD...NONE.
VA...NONE.
WV...NONE.
MARINE...SMALL CRAFT ADVISORY FROM MIDNIGHT TONIGHT TO 6 PM EDT MONDAY
     FOR ANZ530>543.

&&

$$
SYNOPSIS...BAJ
NEAR TERM...BAJ
SHORT TERM...BAJ
LONG TERM...DFH
AVIATION...BAJ/DFH
MARINE...BAJ/DFH
TIDES/COASTAL FLOODING...BAJ/DFH






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