976
FXUS61 KLWX 171427
AFDLWX
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE BALTIMORE MD/WASHINGTON DC
1027 AM EDT THU SEP 17 2026
WHAT HAS CHANGED
SHOWERS FORMED ALONG THE CHESAPEAKE BAY EARLIER BUT ARE NOW
DEPARTING TO THE EAST. OVER THE PAST HOUR, CONVECTION BLOSSOMED
IN NORTH CENTRAL WEST VIRGINIA AND IS MOVING INTO WESTERN
MARYLAND. ASSUMING A STEADY STATE, SHOWERS AND THUNDERSTORMS
WOULD ARRIVE TO THE I-81 CORRIDOR AROUND 1230 TO 1 PM AND THE
METRO AREAS AFTER 230 PM. THUS ANY NEAR TERM ADJUSTMENTS TO THE
FORECAST WILL MOSTLY REVOLVE AROUND POPS.
KEY MESSAGES
- 1) WARM AND HUMID, WITH THUNDERSTORM CHANCES TODAY BEFORE
TURNING COOLER INTO THE FIRST HALF OF THE WEEKEND.
- 2) UNSETTLED WEATHER CONDITIONS RETURN AGAIN SUNDAY AND INTO
NEXT WEEK.
DISCUSSION
KEY MESSAGE 1...WARM AND HUMID, WITH THUNDERSTORM CHANCES TODAY
BEFORE TURNING COOLER INTO THE FIRST HALF OF THE WEEKEND.
RETURN FLOW AROUND THE WEST SIDE OF HIGH PRESSURE IN THE WESTERN
ATLANTIC AND SOUTHEASTERN STATES HAS YIELDED A MILDER AND MORE
MOISTURE-LADEN AIR MASS. UNLIKE PREVIOUS DAYS, DIURNAL SHOWERS
AND THUNDERSTORMS WILL BE POSSIBLE. THE CURRENT PLAYERS ARE:
(1) AN EXPANSIVE AND ANOMALOUS MID/UPPER RIDGE STRETCHING ACROSS
SOUTHERN U.S./GULF COAST UP INTO THE OHIO VALLEY AND MID-
ATLANTIC STATES, (2) A SHARPENING TROUGH CROSSING WESTERN
ONTARIO, AND (3) AN AREA OF DISTURBED WEATHER JUST NORTH OF THE
BAHAMAS. IN TIME, THE HIGH- LATITUDE TROUGH IS EXPECTED TO ERODE
THE NORTHERN EXTENT OF THE RIDGE. ALTHOUGH HEIGHT ANOMALIES
REMAIN AROUND 1.5 TO 2 SIGMA ABOVE MID-SEPTEMBER CLIMATOLOGY,
THE SUBTLE HEIGHT FALLS SHOULD BE ENOUGH TO SUPPORT INCREASED
LIFT THIS AFTERNOON. THIS SHIFT IN THE SYNOPTIC PATTERN ALLOWS A
STALLED FRONT ALONG I-70 IN OHIO/INDIANA TO PUSH SOUTHEASTWARD
LATER TODAY. THE COLD FRONT IS NOT EXPECTED TO CROSS THE ENTIRE
REGION UNTIL LATE TONIGHT INTO EARLY FRIDAY MORNING. THUS, THE
PRE-FRONTAL AIR MASS WILL SUPPORT SCATTERED CONVECTION THIS
AFTERNOON AND EVENING.
WITH HIGH TEMPERATURES RISING INTO THE MID 80S TO LOW 90S,
COUPLED WITH DEW POINTS REACHING THE UPPER 60S, AROUND 1,500 TO
2,000 J/KG OF SURFACE-BASED CAPE DEVELOPS. DEEP-LAYER SHEAR IS
SOMEWHAT MEAGER, GENERALLY AVERAGING AROUND 15 TO 20 KNOTS.
HOWEVER, FORECAST SOUNDINGS DO SHOW SOME MODEST DOWNDRAFT CAPE
AROUND 500 TO 700 J/KG. THIS WOULD BE ENOUGH TO PRODUCE SPOTTY
GUSTY WINDS IN ANY MORE ROBUST THUNDERSTORM. ALTHOUGH LOW-LEVEL
LAPSE RATES SIT IN THE 8 TO 8.5 C/KM RANGE, MID-LEVEL LAPSE
RATES ARE WEAK WHICH CASTS A DOUBT ON HOW TALL ANY STORM WILL
GET. CONVECTION IS EXPECTED TO EITHER PERSIST FROM ONGOING
ACTIVITY AND/OR DEVELOP OFF THE HIGHER TERRAIN BEFORE PUSHING
TOWARD THE I-95 CORRIDOR DURING THE SECOND HALF OF THE AFTERNOON
INTO THE EARLY EVENING. AN INSTABILITY GRADIENT REMAINS IN PLACE
WITH DRIER AIR TO THE SOUTH AND EAST OF WASHINGTON DC, SO
CONVECTION MAY TEND TO SHOW A WEAKENING TREND AS IT PUSHES
TOWARD THE I-95 CORRIDOR. SHOWER CHANCES SHOULD DIMINISH BY A
COUPLE HOURS AFTER DARK GIVEN THE LOSS OF INSTABILITY.
THE COLD FRONT SWEEPS THROUGH OVERNIGHT BEFORE REACHING THE
NORTHERN NECK OF VIRGINIA BY MIDDAY FRIDAY. WITH SOME RESIDUAL
FORCING CROSSING THE CENTRAL VIRGINIA PIEDMONT BACK INTO THE
POTOMAC HIGHLANDS, SOME AFTERNOON SHOWERS AND THUNDERSTORMS ARE
POSSIBLE. OVERALL, FORECAST HIGHS WILL BE IN THE UPPER 70S TO
MID 80S, WITH LOW/MID 70S FOR MOUNTAIN LOCALES. WINDS WILL
MAINLY BE OUT OF THE NORTH WITH GUSTS UP TO 15 TO 20 MPH. A
COOLER NIGHT LIES AHEAD AS HIGH PRESSURE CROSSES SOUTHERN QUEBEC
INTO NEW ENGLAND. FRIDAY NIGHT'S LOWS SHOULD BE IN THE UPPER 50S
TO MID 60S. THE INFLUENCE OF THIS RIDGE FAVORS NEAR AVERAGE
TEMPERATURES TO START THE WEEKEND ON SATURDAY. ALTHOUGH SKIES
SHOULD BE MOSTLY CLOUDY, IT IS A MAINLY DRY FORECAST WITH HIGHS
IN THE 70S TO LOW 80S WITH A REDUCTION IN HUMIDITY.
KEY MESSAGE 2...UNSETTLED WEATHER CONDITIONS RETURN AGAIN SUNDAY
AND INTO NEXT WEEK.
FROM SUNDAY AND INTO NEXT WEEK, MODEL GUIDENCE POINTS TO MORE
UNSETTLED WEATHER, AS A PERSISTENT UPPER RIDGE AXIS PUSHES TOWARDS
THE DEEP SOUTH. THIS ALLOWS MULTIPLE UPPER-LEVEL DISTURBANCES TO
MOVE ACROSS THE MID-ATLANTIC FROM THE GREAT PLAINS, DRIVING DAILY
CHANCES FOR SHOWERS AND THUNDERSTORMS. LONG-TERM ENEMBLE MODELS SHOW
A WIDE RANGE OF POSSIBILITIES REGUARDING THE TIMING AND LOCATION OF
THESE DISTURBANCES. WHILE WIDESPREAD CHANCES FOR SHOWERS AND
THUNDERSTORMS EXIST, THE EXACT TIMING, COVERAGE, AND NATURE OF THESE
STORMS REMAINS QUITE UNCERTAIN. IT WILL INTERESTING TO WATCH THE
EVOLUTION OF THESE DISTURBANCES IN THE COMING MODEL RUNS.
AVIATION /14Z THURSDAY THROUGH MONDAY/
CANADIAN HIGH PRESSURE HAS BEGUN TO PULL JUST OFF THE ATLANTIC
COAST WHICH HAS RESULTED IN A SHIFT TO LIGHT SOUTH TO
SOUTHEASTERLY WINDS. SOME SHOWERS AND THUNDERSTORMS EMERGE OFF
THE HIGHER TERRAIN THIS AFTERNOON. THE THREAT FOR THUNDER DOES
DROP OFF WITH EASTWARD EXTENT OF THESE STORMS, THOUGH THERE IS
STILL SOME RISK FOR A THUNDERSTORM AT ANY TERMINAL. IF ACTIVITY
OVER THE APPALACHIAN MOUNTAINS MAINTAINS A STEADY STATE, IT
WOULD BE TIMED INTO MRB AROUND 17Z AND THE METRO AREAS BETWEEN
18-20Z. ANY RESTRICTIONS SHOULD BE LIMITED TO SUCH CONVECTION
BEFORE VFR CONDITIONS RETURN INTO TONIGHT.
WINDS SHIFT TO NORTHERLY ON FRIDAY BEHIND AN EARLY MORNING COLD
FRONT. AFTERNOON GUSTS COULD REACH 15 TO 20 KNOTS DEPENDING ON
THE CALIBER OF BOUNDARY LAYER MIXING. SOME SHOWERS OVER CENTRAL
VIRGINIA MAY IMPACT KCHO, BUT CONFIDENCE IS LOW AT THIS TIME. AS
HIGH PRESSURE PASSES OFF TO THE NORTH, WINDS TURN EAST TO
SOUTHEASTERLY ON SATURDAY, BUT WITH CONTINUED VFR CONDITIONS.
SOUTHERLY WINDS ON SUNDAY WITH GUSTS UP TO 15-25 KNOTS ARE
POSSIBLE. IN GENERAL, EXPECT PREDOMINANTLY VFR CONDITIONS TO
PERSIST, BUT FLIGHT CONDITIONS COULD DETERIORATE AS SHOWER AND
THUNDERSTORM POTENTIAL INCREASE LATE IN THE WEEKEND AND INTO
EARLY NEXT WEEK. WEATHER CONDITIONS EARLY NEXT WEEK REMAIN
UNCERTAIN.
MARINE
WITH CANADIAN HIGH PRESSURE EDGING OFF THE ATLANTIC COAST, A
RETURN SOUTH TO SOUTHEASTERLY FLOW IS EVIDENT IN MARINE
OBSERVATIONS THIS MORNING. AHEAD OF THE COLD FRONT MOVING IN
FROM THE OHIO VALLEY, A FURTHER UPTICK IN THESE SOUTH TO
SOUTHEASTERLY WINDS IS POSSIBLE DURING THE AFTERNOON/EVENING
HOURS. THIS MAY EVEN NEAR SMALL CRAFT ADVISORY LEVELS,
ESPECIALLY ACROSS THE WIDER CHESAPEAKE BAY, WHERE AN ADVISORY IS
IN EFFECT FROM 3 UNTIL 11 PM TODAY. A FEW STORMS COULD REACH
THE MARINE WATERS DURING THE LATE AFTERNOON TO EVENING, BUT
CONFIDENCE IS LOW WHETHER THESE WOULD NECESSITATE SPECIAL MARINE
WARNINGS.
WINDS SHIFT TO NORTHERLY ON FRIDAY MORNING BEHIND THE COLD
FRONT. SOME NORTHERLY CHANNELING EFFECTS ARE POSSIBLE ON FRIDAY
MORNING INTO THE AFTERNOON DUE TO THE FRONTAL SURGE. SMALL CRAFT
ADVISORIES MAY BE NEEDED FOR SUCH AN EVOLUTION. AS HIGH PRESSURE
PASSES BY TO THE NORTH, WINDS SHIFT FROM NORTHEASTERLY TO EAST-
SOUTHEASTERLY ON SATURDAY. THIS COULD BRING A FEW GUSTS TO NEAR
20 KNOTS WHICH COULD APPROACH ADVISORY LEVELS.
SOUTHERLY WINDS ON SUNDAY WITH GUSTS OVER THE WATERS MEETING OR
SURPASING SCA LEVELS ARE POSSIBLE THROUGH MONDAY. WEATHER CONDITIONS
EARLY NEXT WEEK REMAIN UNCERTAIN.
TIDES/COASTAL FLOODING
WITH WINDS REMAINING OUT OF THE SOUTH TO SOUTHEASTERLY DIRECTION,
THIS FLOW REGIME HAS KEPT TIDAL ANOMALIES AROUND 0.50 TO 0.75
FEET. THIS HAS BEEN ENOUGH TO PUSH A FEW OF THE MORE SENSITIVE
LOCATIONS INTO THE ACTION STAGE. UNTIL THE COLD FRONT MOVES
THROUGH EARLY FRIDAY, WATER LEVELS WILL REMAIN SOMEWHAT
ELEVATED. ALTHOUGH NO TIDAL SITE IS SHOWN TO EXPERIENCE ANY
COASTAL FLOODING, IT WILL NEED TO BE MONITORED. THIS IS
PARTICULARLY THE CASE AT ANNAPOLIS, ESPECIALLY DURING THE HIGHER
OF THE TWO ASTRONOMICAL HIGH TIDES. EXPECT ANOMALIES TO DROP ON
FRIDAY INTO SATURDAY BEHIND THE MENTIONED COLD FRONT.
LWX WATCHES/WARNINGS/ADVISORIES
DC...NONE.
MD...NONE.
VA...NONE.
WV...NONE.
MARINE...SMALL CRAFT ADVISORY FROM 3 PM THIS AFTERNOON TO 11 PM EDT
THIS EVENING FOR ANZ532>534-543.
DISCUSSION...ADS/BRO/KVH
AVIATION...ADS/BRO/KVH
MARINE...ADS/BRO/KVH
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