Heavy Rain in Idaho, March 30, 2006

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SPOKANE WA
229 PM PST THU MAR 30 2006

.SYNOPSIS...A SERIES OF LOW PRESSURE SYSTEMS WILL SEND MOISTURE
INTO THE INLAND NORTHWEST THROUGH THE WEEKEND RESULTING IN
UNSETTLED CONDITIONS AT TIMES.

&&

.DISCUSSION...
TONIGHT AND FRIDAY...AN UPPER LEVEL TROUGH OFF THE COAST NEAR 135W
IS RESULTING IN A SOUTHWEST FLOW OVER THE INLAND NW. WATER VAPOR
SATELLITE SHOWS ABUNDANT UPPER LEVEL MOISTURE BEING TRANSPORTED
INTO THE AREA...WHICH WILL PERSIST INTO FRIDAY. A SHORT WAVE
LOCATED OFF THE NORTHERN CA/SOUTHERN OR COAST WILL ROTATE NORTH-NORTHEAST
TONIGHT. IN ADDITION...FAVORABLE UPPER LEVEL JET SUPPORT WILL
SUPPORT FAVORABLE CONDITIONS FOR LOWERING OF CLOUD BASES WITH
SHOWER COVERAGE INCREASING OVERNIGHT INTO FRIDAY MORNING. NUMEROUS
SHOWERS WILL DEVELOP ALONG THE EAST SLOPES OF THE CASCADES AND
ACROSS THE HIGHER TERRAIN OF NORTHERN WASHINGTON AND IDAHO ON
FRIDAY WHERE UPSLOPE FLOW WILL OCCUR...WITH SCATTERED SHOWERS
EXPECTED ELSEWHERE. TEMPERATURES AT MOST LOCATIONS WILL BE
SLIGHTLY COOLER ON FRIDAY DUE TO ABUNDANT CLOUD COVER LIMITING
INCOMING SOLAR RADIATION. THE GFS MODEL CONTINUES TO HINT AT
MARGINAL INSTABILITY FOR A POSSIBLE THUNDERSTORM MAINLY OVER THE
BLUE MOUNTAINS ON FRIDAY...BUT CLOUD COVER SHOULD LIMIT SURFACE
HEATING SO WILL LEAVE OUT OF FORECAST. JW

FRI NT THROUGH SUNDAY...FCST FOCUS WILL BE ON THE EJECTION INLAND
OF THE UPPER TROF ALONG THE PAC NW COAST...EJECTED BY A STRONGER WAVE
DIGGING SE OUT OF THE GULF OF ALASKA. AS WITH ALL DISCONTINUOUS
RETROGRESSION PATTERNS OF THIS TYPE...TIMING OF EJECTING WAVE IS
KEY AND, OF COURSE, THE PRIMARY CHALLENGE. GIVEN ITS CONSISTENCY,
WEVE GENERALLY FOLLOWED THE GFS TIMING AND RATE OF FILLING OF
THIS LIFTING VORT MAX AS IT TRACKS NE THRU ERN WA AND N ID. THE
MOST LIKELY TRACK OF THE VORT MAX AND UPPER TROF AXIS WILL BE
ACROSS ORE SAT, THEN SHEARING AND WEAKENING RAPIDLY OVER N ID SAT
NT. THE ABOVE-MENTIONED SCENARIO SHOULD PRODUCE A BAND OF SHOWERS
AND, GIVEN THE DEGREE OF INSTABILITY BENEATH H5 TEMPS OF -25C TO
-29C, ISOLATED EMBEDDED THUNDERSTORMS FOR MOST ZONES BY SAT
AFTERNOON. THERE ARE STRONG INDICATIONS THAT THE UPPER TROF WILL
SPLIT, WITH THE ERN, AND STRONGEST, LOBE OF THE VORT MAX MOVING E
INTO SRN ID, AND THE WEAKER MAX LIFTING NE THRU ERN WA. REGARDLESS
OF THE EXACT TIMING AND DEGREE OF SHEARING OF THE UPPER TROF, THE
MAIN MESSAGE WILL BE A DECENT SHOT OF SHOWERS AND ISOLD TSRA, WITH
SNOW ACCUMULATION LEVELS (SNOW PELLET/GRAUPEL FOR NON-HAILERS)
DOWN TO AS LOW AS 3.5K FT MSL IN THE HEAVIEST SHOWERS (4.5K FT MSL
FOR THE REST). RAPID STABILIZATION SAT NT IN THE WAKE OF THIS WAVE
SHOULD END THE SIGNIFICANT SHOWER THREAT BY 6Z. A MORE STRATIFORM-
TYPE PCPN EVENT WILL DEVELOP SHORTLY THEREAFTER (BY MIDDAY SUNDAY)
AS THE NEXT PACIFIC COLD-FRONT MAKES LAND-FALL AND MOVES ACROSS
THE CASCADES. BZ

SUNDAY NIGHT THROUGH THURSDAY...LONGWAVE TROF WITH AXIS PLACEMENT
REMAINING EAST OF 140W CONTINUES TO BE BOTH THE ORIGIN AND STEERING
MECHANISM FOR SYNOPTIC AND SMALLER SCALE SHORTWAVE DISTURBANCES THAT
AFFECT EASTERN WASHINGTON AND NORTHERN IDAHO THROUGH THIS TIME
PERIOD. NESTED IN THE CENTER OF THE TROF IS A FAIRLY COLD AND
CONDITIONALLY UNSTABLE AIRMASS...EVIDENT FROM SATELLITE IMAGERY
SHOWING ABUNDANT OPEN CELLULAR CUMULUS RESIDING IN LARGE PARTS OF IT.
LATEST GFS RUNS DEPICT NICELY BUILT LOW PRESSURE SYSTEM NESTED INSIDE
THE LONGWAVE TROF SWEEPING A FRONTAL ZONE SPIRALING OUTWARD
FROM THE LOW AND INTO THE FORECAST AREA SUNDAY NIGHT. THE BAND
DOESNT REALLY GO AWAY BUT RATHER STALLS OUT OVERHEAD AND WILL ACT AS
A FOCUSING MECHANISM ALLOWING A CHANCE OF PRECIPITATION TO OCCUR IN
ITS VICINITY. THE PARENT SYNOPTIC SCALE CLOSED LOW EVENTUALLY
TAKES A PATH TO THE SOUTH AND BECOMES MORE A CALIFORNIA ISSUE THAN
ONE FOR THE PACIFIC NORTHWEST TUESDAY. MOISTURE TAP UP INTO MONDAY
NIGHT FOR THE EARLIER DISCUSSED BAROCLINIC BAND STILL APPEARS TO DRAW
FROM TWO SOURCES...THE PACIFIC VIA A ROUTE THROUGH BAJA OVER NEVADA
WITH A SOUTHERLY FLOW AND FROM GULF OF MEXICO VIA A ROUTE THROUGH
TEXAS...PER EXAMINATION OF 300K AND 290K ISENTROPIC SURFACES AND
800MB AND 700MB THETA-E ANALYSIS. WEDNESDAY AND THURSDAY LOOK TO BE
THE DRIER PART OF THE FORECAST WITH ALL THE ENERGY DIVING SOUTH AS
THE CLOSED LOW BECOMES A CALIFORNIA ISSUE WITH EASTERN WASHINGTON
AND NORTHERN IDAHO ENDING UP IN THE MIDDLE OF A SPLIT FLOW OR EVEN
UNDER INFLUENCE OF SHORTWAVE RIDGING PASSING THROUGH THE NORTHERN
BRANCH OF THE SPLIT. FORECAST TEMPS DONT STRAY TOO FAR FROM WHAT IS
CONSIDERED NORMAL FOR THIS TIME OF YEAR GIVEN THIS UNSETTLED WEATHER
SCENARIO WHICH IS QUITE TYPICAL OF SPRING FOR EASTERN WASHINGTON AND
NORTHERN IDAHO.

&&

.AVIATION...
PRIMARILY VFR CONDITIONS WITH SPOTTY SHOWERS IN THE
PERIPHERY OF LOCAL TERMINALS. TONIGHT A WEATHER DISTURBANCE WILL
MOVE UP INTO THE AREA FROM THE SOUTH AND SOUTHWEST BRINGING
SCATTERED SHOWERS AND SPOTTY MVFR CONDITIONS STARTING AT ABOUT
6Z NEAR THE CASCADES...TO ABOUT 12Z FOR THE SPOKANE AREA. SHOWERS
LINGER ON IN AVIATION FORECASTS FOR A TIME. PRECIP CHANCE MOST
LIKELY TO DECREASE SUBSTANTIALLY OVER THE LOWLANDS EAST OF THE
CASCADE CREST AND INTO THE COLUMBIA BASIN AT ABOUT 22Z OR SO.

&&

.PRELIMINARY POINT TEMPS/POPS...
SPOKANE        39  52  40  55  34  55 /  30  70  40  50  40  30
COEUR DALENE  38  51  38  55  36  54 /  30  60  40  50  40  30
PULLMAN        40  54  41  52  35  56 /  20  50  20  40  40  20
LEWISTON       43  59  44  53  38  60 /  20  50  20  40  40  10
COLVILLE       38  50  38  60  33  56 /  30  70  50  60  50  50
SANDPOINT      36  49  36  56  33  56 /  30  60  70  60  50  50
KELLOGG        37  52  38  57  35  53 /  30  60  50  60  60  30
MOSES LAKE     41  60  42  57  33  61 /  20  50  20  20  20  40
WENATCHEE      44  61  42  58  37  60 /  40  50  20  20  20  50
OMAK           40  56  40  59  34  60 /  40  70  30  40  30  60

&&

.OTX WATCHES/WARNINGS/ADVISORIES...
ID...NONE.
WA...NONE.
&&