400 
FXUS02 KWBC 171816
PMDEPD

EXTENDED FORECAST DISCUSSION FOR 20 OCT THRU 22 OCT 1998
HYDROMETEOROLOGICAL PREDICTION CENTER..NCEP...NWS...WASHINGTON DC 
2:15 PM EDT SAT 17 OCT 1998

             ...MODEL DISCUSSION AND OVERVIEW...

LATEST MEDIUM-RANGE MODEL D+3 500MB MEANS CONTINUE TO FORECAST A
MAJOR LONGWAVE CHANGE OVER THE ERN PACIFIC AND NOAM. A NEAR
REVERSAL OF THE RECENT E CENTRAL PACIFIC RIDGE/W COAST TROF PATTERN
IS DUE. HEIGHTS ARE FORECAST TO FALL STRONGLY NEAR 45N/160W...AND RISE
STRONGLY OVER THE PACIFIC NW/VANCOUVER/BC. SIGNIFICANT HEIGHT FALLS
ARE ALSO EXPECTED OVER THE GREAT LAKES IN RESPONSE TO THE UPSTREAM
AMPLIFICATION. A  PIECE OF LEFTOVER VORT WILL BE TRAPPED UNDERNEATH 
THE STRONG RIDGE IN THE PACIFIC NW STATES...GENERATING SW FLOW ALOFT
THAT WILL OVERRUN A COOL DOME OF SFC HIGH PRES IN TX/ERN NM MON-
TUE.  HIGH LEVEL MOISTURE FROM MADELINE MAY ENHANCE OVERRUNNING
PCPN IN SWRN TX AND SERN NM. 

THE MOST IMPORTANT MODEL DIFFERENCES TODAY CENTER ON THE DETAILS
OF THE AMPLIFYING MEAN TROF OVER THE GREAT LAKES AREA WED-THU. FOR
WED...THE ECMWF/NAVY NOGAPS ARE STRONGEST WITH THIS NEW TROF AND
DROP A VERY STRONG VORT MAX ACROSS THE UPPER MIDWEST AND MID MS
VLY...RESPECTIVELY. BY FRI...THE NAVY NOGAPS/UKMET ARE THE STRONGEST
OF THE MODELS...SHOWING A DEEP ELONGATED TROF/CLOSED LOW FROM THE
LOWER OH VLY TO THE LOWER LAKES. THE ECMWF/MRF FOR THU DO NOT DIG
THE UPPER TROF AS FAR S AS THE NOGAPS/UKMET...SHOWING THEIR HIGHEST
VORT VALUES FROM THE OH VLY TO THE MID-ATLANTIC. A BLEND OF MODEL
FORECASTS FOR WED-THU IS PROBABLY OUR COURSE OF LEAST REGRET. WE
WONT BE ABLE TO REFINE THE DETAILS OF THIS DIGGING SHORTWAVE...
UNTIL IT DROPS DOWN THE E SIDE OF THE BUILDING LONGWAVE RIDGE ALONG
THE W COAST OF CANADA IN ABOUT TWO DAYS.
     
....REGIONAL HIGHLIGHTS....

...SWRN STATES/TX...

A COOL DOME OF HIGH PRES IN TX WILL BE OVERRUN BY SW FLOW
ALOFT...WHICH MAY BE TAPPING MOISTURE FROM TROPICAL SYSTEMS
MADELINE OR EVEN LESTER ON THE PACIFIC SIDE OF THE MEXICAN COAST.  
SOME SOAKING LOCALLY HEAVY RAINFALL IS POSSIBLE MON-TUE IN SW
TX/SERN NM...MORE FROM A PROLONGED PERIOD OF MODERATE STRATIFORM
RAIN THAN FROM INTENSE CONVECTIVE DOWNPOURS. THE ECMWF/MRF
LEAVED A WEAK CLOSED UPPER LOW TRAPPED VCNTY OF SRN CA/AZ THRU THE
MEDIUM RANGE. THE NAVY NOGAPS/ECMWF WEAKEN THIS CLOSED UPPER LOW
LATE WED INTO THU...MOVING IT NWD INTO NV. WE DO EXPECT SOME DRIER
AIR TO THE REAR OF THE DIGGING UPPER TROF IN THE MID/UPPER MS VLY
WED-THU TO FILTER S INTO TX/ERN NM...BRINGING A GRADUAL END TO PCPN
BY THU. THE NAVY NOGAPS/UKMET...DIGGING THIS TROF FARTHER S INTO THE
WRN GULF STATES THAN THE MRF/ECMWF...IMPLY MORE RAPID DRYING FOR
TX/NM...WITH PCPN ENDING WED.        

...REMAINDER OF THE WRN HALF OF THE COUNTRY..

THIS PORTION OF THE NATION WILL HAVE MOSTLY DRY WEATHER UNDER THE
INFLUENCE OF THE UPPER RIDGE. TEMPS WILL START OUT BELOW NORMAL
BUT MODERATE WITH TIME. THE PACIFIC COAST MAY SEE WELL ABOVE
NORMAL MAXIMA BY WED AS SURFACE PRES BUILD INTO THE GREAT BASIN
AND STRONG SANTA ANNA GRADIENTS DEVELOP.


...ERN HALF OF THE US...

TEMPERATURES WILL GET COLDER WITH TIME AS UPPER HEIGHTS FALL OVER
THE GREAT LAKES. THE PCPN FORECAST IS PROBLEMATIC...DEPENDING ON
HOW MUCH HEIGHTS FALL OVER THE LAKES/APLCNS BY THE END OF THE
PERIOD. A CONSENSUS OF MODELS SHOWS THICKNESSES FALLING TO BELOW
5400M OVER THE GREAT LAKES LATER TUE. THIS WILL GENERATE SOME
INSTABILITY SHOWERS...BUT BECAUSE OF THE WARM LAKES...THESE SHOULD
BE RAIN RATHER THAN SNOW. DEPENDING ON THE AMPLITUDE AND SRN
EXTENT OF THE GREAT LAKES TROF...PCPN MAY DEVELOP IN THE MID
ATLANTIC-NEW ENG AREAS BY THU. THE ECMWF AND NAVY NOGAPS BEST
LEAD INTO A COASTAL CYCLOGENESIS SCENARIO...SINCE THEY ALREADY HAVE
A SURFACE LOW IN THE ERN LAKES WED. SOME SHOWERS MAY LINGER OVER
PENINSULAR FL THRU THE PERIOD. 

FLOOD/FORECAST OPERATIONS BRANCH

GRAPHICS AVAILABLE ON THE WEB AT http://www.ncep.noaa.gov/HPC