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Area Forecast Discussion

823
FXUS64 KHUN 281121 AAA
AFDHUN

AREA FORECAST DISCUSSION...UPDATED
NATIONAL WEATHER SERVICE HUNTSVILLE AL
621 AM CDT SAT MAR 28 2015

.UPDATE...
FOR 12Z TAFS

&&

.DISCUSSION...
/ISSUED 603 AM CDT SAT MAR 28 2015/
A LONGWAVE TROUGH CONTINUES TO ENCOMPASS EASTERN NORTH AMERICA THIS
MORNING...AND WITH A SUBTROPICAL RIDGE CENTERED ACROSS THE
SOUTHWESTERN DESERTS AND NW MEXICO THE SYNOPTIC PATTERN WILL MAINTAIN
STRONG NORTHWEST FLOW ALOFT OVER THE LOCAL AREA. EMBEDDED WITHIN THE
BROADER LONGWAVE TROUGH...A MID-LEVEL VORT MAX WHICH WAS RESPONSIBLE
FOR LIGHT PRECIPITATION ACROSS THE REGION FRIDAY/FRIDAY EVENING WILL
CONTINUE TO SHIFT EASTWARD OFF THE COAST OF THE CAROLINAS AND
VIRGINIA THIS MORNING WHILE A WEAKER UPSTREAM DISTURBANCE DIGS
SOUTHEASTWARD INTO THE MID-MISSISSIPPI VALLEY REGION. THE OVERALL
SYNOPTIC SETUP DOES NOT APPEAR AS FAVORABLE FOR PRECIPITATION
TODAY...AS 1) THE WAVE IS FORECAST TO WEAKEN AS IT SPREADS ACROSS THE
REGION TODAY AND 2) FORECAST SOUNDINGS SUGGEST A MUCH DRIER LOW-
LEVEL AIRMASS WHICH WILL NEED TO BE SATURATED TO GENERATE
PRECIPITATION. THAT SAID...A FEW LIGHT SHOWERS WILL STILL BE POSSIBLE
ESPECIALLY ACROSS NW ALABAMA -- WHERE REGION WILL BE FAVORABLY
LOCATED BENEATH LEFT EXIT REGION OF 80-90 KT UPPER SPEED MAX.
ALTHOUGH NOT INCLUDED IN THE OFFICIAL GRIDS...VERY STEEP LAPSE RATES
BETWEEN THE SURFACE-850 MB WILL ENHANCE CONVECTIVE INSTABILITY AND
MAY ALLOW SOME ICE PELLETS TO BRIEFLY MIX IN THE WITH ANY
PRECIPITATION THAT OCCURS IN THE REGION.

CONDITIONS WILL IMPROVE THIS EVENING...AS THE LONGWAVE TROUGH AXIS
SHIFTS EASTWARD ACROSS THE EASTERN SEABOARD AND SHORTWAVE RIDGING
BECOMES ESTABLISHED BETWEEN THE DEPARTING SYSTEM AND AN UPSTREAM
TROUGH SPREADING SOUTHEASTWARD ACROSS THE NORTHERN PLAINS. THIS WILL
ALLOW SKIES TO CLEAR RATHER QUICKLY AFTER SUNSET...WITH TEMPERATURES
EXPECTED TO FALL QUICKLY WITHIN LIGHT FLOW REGIME PROVIDED BY SURFACE
RIDGE OVER THE CENTRAL APPALACHIANS. MODELS SUGGEST THAT SURFACE
DEWPOINTS WILL RANGE FROM THE LOWER/MID-TEENS IN THE NORTHEAST TO THE
UPPER TEENS/LOWER 20S IN THE SOUTHWEST...WHICH WOULD ORDINARILY
PROMOTE A HARD FREEZE. HOWEVER...LIGHT EAST-SOUTHEASTERLY RETURN
FLOW WILL BEGIN BETWEEN 06-12Z SUNDAY LIMITING TEMPERATURE FALLS
DURING THIS PERIOD. THE CURRENT FREEZE WARNING WILL REMAIN IN EFFECT
FROM 11 PM THIS EVENING THROUGH 9 AM SUNDAY...WITH STRENGTHENING LOW-
LEVEL SOUTHERLY WINDS AND HEIGHT RISES ALOFT LEADING TO MUCH WARMER
CONDITIONS ON SUNDAY.

GLOBAL MODEL GUIDANCE IS IN AGREEMENT THAT THE PREVIOUSLY MENTIONED
NORTHERN PLAINS TROUGH WILL BEGIN TO INTERACT WITH A STRONGER
TROUGH DIGGING INTO THE WESTERN GREAT LAKES ON SUNDAY...WITH THE
CONSOLIDATED SURFACE LOW TRACKING EASTWARD ACROSS ONTARIO/QUEBEC ON
SUNDAY NIGHT. A TRAILING COLD FRONT WILL EXTEND FROM THE SOUTHWESTERN
GREAT LAKES INTO THE SOUTHERN PLAINS AT 00Z MONDAY... AND IS FORECAST
TO SHIFT STEADILY SOUTHEASTWARD THROUGH THE NIGHT AND ROUGHLY BISECT
THE FORECAST AREA AT 12Z MONDAY. LIFT FROM STRONG LOW- LEVEL
WARM/MOIST ADVECTION WILL SUPPORT THE DEVELOPMENT OF SEVERAL
ORGANIZED CLUSTERS OF CONVECTION ALONG THE FRONT ACROSS WESTERN
TENNESSEE/NORTHERN MISSISSIPPI ON SUNDAY EVENING...WITH THIS ACTIVITY
EXPECTED TO BE DRIVEN SOUTHEASTWARD INTO THE REGION BY STRENGTHENING
NORTHWEST FLOW ALOFT. ALTHOUGH FORECAST SOUNDINGS STILL SUGGEST THAT
THIS CONVECTION WILL BE ELEVATED...DEEP-LAYER SHEAR OF 45-55 KTS AND
SUFFICIENTLY STEEP LAPSE RATES IN THE 700-500 MB LAYER MAY PROMOTE
SOME SMALL HAIL WITH THE STRONGER CELLS. THUNDERSTORM ACTIVITY
BETWEEN 12-18Z WILL BE CONFINED TO LOCATIONS ALONG AND SOUTH OF THE
COLD FRONT -- WHICH SHOULD MOVE ENTIRELY SOUTH OF THE FORECAST AREA
BY 18Z.

IN THE EXTENDED PERIOD...A NORTHWEST FLOW ALOFT WILL PREVAIL ACROSS
THE REGION DURING THE PERIOD FROM MONDAY NIGHT-TUESDAY NIGHT. COOL
CONDITIONS ARE EXPECTED IN THE WAKE OF THE FRONT ON MONDAY NIGHT AS
A MODIFIED PACIFIC AIRMASS BUILDS INTO THE REGION. HOWEVER...MODELS
INDICATE THAT LOW-LEVEL MOISTURE WILL RETURN TO THE REGION RATHER
QUICKLY ON TUESDAY AS A WARM FRONT LIFTS NORTHWARD. THIS WILL RESULT
IN AN INCREASE IN SHOWER/THUNDERSTORM ACTIVITY ONCE AGAIN...ALTHOUGH
WILL LEAVE POPS IN THE LOW CHANCE RANGE AS MID-LEVEL FORCING APPEARS
TO BE WEAK. A SOUTHERN STREAM DISTURBANCE WILL LIFT NORTHEASTWARD
ACROSS THE LOWER MISSISSIPPI VALLEY ON WEDNESDAY AND BEGIN TO IMPACT
OUR WEATHER CONDITIONS WITH AN EVEN HIGHER CHANCE FOR SHOWERS/STORMS
ON WEDNESDAY NIGHT/THURSDAY. THERE ARE STILL SIGNIFICANT DIFFERENCES
IN THE ECMWF/GFS W.R.T. A STRONGER NORTHERN STREAM TROUGH LATE NEXT
WEEK. WE HAVE MAINTAINED A LOW POP FOR SHOWERS/STORMS UNTIL SPEED OF
THIS SYSTEM AND ITS ASSOCIATED FRONTAL PASSAGE ARE IN BETTER
AGREEMENT.

70/DD

&&

.AVIATION...
FOR 12Z TAFS...EVEN WITH MID/HIGH CLOUDS TRAVERSING SEWD ACROSS THE
REGION...VFR CONDS ARE XPCTED THROUGH THE TAF PERIOD.

09

&&

.HUN WATCHES/WARNINGS/ADVISORIES...
AL...FREEZE WARNING UNTIL 10 AM CDT THIS MORNING FOR ALZ001>010-016.

     FREEZE WARNING FROM 11 PM THIS EVENING TO 9 AM CDT SUNDAY FOR
     ALZ001>010-016.

TN...FREEZE WARNING UNTIL 10 AM CDT THIS MORNING FOR TNZ076-096-097.

     FREEZE WARNING FROM 11 PM THIS EVENING TO 9 AM CDT SUNDAY FOR
     TNZ076-096-097.

&&

$$

FOR MORE INFORMATION PLEASE VISIT OUR WEBSITE
AT WEATHER.GOV/HUNTSVILLE.



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