SPC AC 171957
DAY 1 CONVECTIVE OUTLOOK
NWS STORM PREDICTION CENTER NORMAN OK
0257 PM CDT SAT OCT 17 2009
VALID 172000Z - 181200Z
...NO SVR TSTM AREAS FORECAST...
...PACIFIC NW...
LATE MORNING FORECAST REMAINS ON TRACK WITH OVERALL EVOLUTION OF
COMPACT/PROGRESSIVE SHORTWAVE TROUGH...NOW LOCATED JUST OFF THE
PACIFIC NW COAST...AND EXPECTED TO REACH SRN ALBERTA BY 12Z SUNDAY.
AT 19Z...SATELLITE IMAGERY SHOWED A MID LEVEL IMPULSE/CIRCULATION
CENTER LOCATED 250 W OTH IN BASE OF THE MAIN TROUGH...WITH TRENDS
SUGGESTING FORCING FOR ASCENT ALONG AND JUST AHEAD OF THIS FEATURE
SHOULD REACH THE NWRN ORE/WRN WA COAST AFTER 21Z AND SPREAD INLAND
THROUGH 00-01Z. INSTABILITY WILL BE ON THE INCREASE AS LAPSE RATES
STEEPEN WITH APPROACH OF UPPER TROUGH/COLD MID LEVEL TEMPERATURES
/-20 C AT 500 MB/. THIS COMBINED WITH 50 KT EFFECTIVE BULK SHEAR
SUGGESTS THAT ANY STORM THAT CAN DEVELOP WITHIN THE POST-FRONTAL
ENVIRONMENT WOULD BE LOW TOPPED WITH ROTATION. THIS OUTLOOK HAS
INTRODUCED A LOW SEVERE HAIL PROBABILITY GIVEN THAT NEW TSTM
DEVELOPMENT WILL BE LOCATED WITHIN A MORE UNSTABLE AIR MASS THAN WAS
PRESENT EARLIER TODAY WITH PASSAGE OF COLD FRONT WHERE A COUPLE OF
HAIL REPORTS /ONE SEVERE/ OCCURRED SSW AND S OF SEA.
THE SEVERE WEATHER THREAT...INCLUDING LOW TORNADO POTENTIAL...
REMAINS CONDITIONAL ON THE DEVELOPMENT OF LOW TOPPED STORMS ALONG
AND AHEAD OF THE AFOREMENTIONED MID LEVEL IMPULSE. SATELLITE
IMAGERY COMBINED WITH OBJECTIVE ANALYSES INDICATED A BROAD ZONE OF
SUBSIDENCE /LACK OF CLOUDS/ BETWEEN THE IMPULSE AND THE EXTENSIVE
AREA OF CLOUDINESS/EMBEDDED CONVECTION FROM JUST OFF THE PACIFIC NW
COAST TO THE CASCADES. ALTHOUGH SURFACE HEATING/DESTABILIZATION IS
POSSIBLE AS THE CLEARING ZONE REACHES FAR NWRN ORE/SWRN WA LATER
THIS AFTERNOON...THE LACK OF LIGHTNING WITH CONVECTION ACCOMPANYING
THE IMPULSE IS RESULTING IN A LOWER CONFIDENCE FOR ENOUGH STORM
DEVELOPMENT TO SUPPORT INTRODUCTION OF A SLIGHT RISK.
..PETERS.. 10/17/2009
.PREV DISCUSSION... /ISSUED 1052 AM CDT SAT OCT 17 2009/
...PACIFIC NW...
A VERY MOIST...WARM CONVEYER BELT AHEAD OF THE STRONG UPPER TROUGH
IS NOW SPREADING CONSIDERABLE PCPN ACROSS WRN WA/NWRN OR. BY MID
AFTERNOON ALL THE MODELS ARE IN BASIC AGREEMENT THAT A STRONG WIND
MAX AND MID LEVEL COLD ADVECTION WILL ACCOMPANY THE TROUGH
INLAND...CHANGING THE CHARACTER OF THE PCPN TO MORE CONVECTIVE IN
NATURE. THIS IS CONFIRMED BY THE OBSERVED LIGHTNING THIS MORNING
OFFSHORE AHEAD OF THE TROUGH AND SURFACE LOW.
THE ACCOMPANYING SURFACE LOW WILL BE MOVING NEWD ACROSS VANCOUVER
ISLAND BY THIS EVENING MAINTAINING A MOIST ONSHORE FLOW OVER WRN
WA/NWRN OR...WHICH COUPLED WITH THE STRONG MID LEVEL COOLING WILL
LEAD TO A MARGINALLY UNSTABLE AIR MASS WITH MLCAPES OF A FEW HUNDRED
J/KG DEVELOPING W OF CASCADES.
GIVEN THE STRONG DEEP LAYER CYCLONIC SHEAR OF 50-60KT...THERE IS THE
POTENTIAL FOR DEVELOPMENT OF LOW TOPPED ROTATING THUNDERSTORMS THIS
AFTERNOON. WITH REGARDS TO ANY TORNADO THREAT...WHILE THE SHEAR AND
POTENTIAL INSTABILITY WOULD SEEM TO BE SUFFICIENT...THE LACK OF
FAVORABLE BOUNDARY LAYER FLOW GIVEN THE SURFACE LOW OFFSHORE IS NOT
PARTICULARLY STRONG...WOULD LIMIT THE EFFECTS OF TERRAIN ENHANCEMENT
WHICH USUALLY PLAYS A ROLE IN THESE TYPE OF SITUATIONS W OF THE
CASCADES.
WILL CONTINUE WITH A LOW PROBABILITY OF ISOLATED TORNADOES...WITH
THE POSSIBILITY OF AN UPGRADE TO A SLIGHT IN THE 20Z OUTLOOK IF
BOUNDARY LAYER CONDITIONS SEEM MORE FAVORABLE THAN NOW EXPECTED.
CLICK TO GET WUUS01 PTSDY1 PRODUCT
NOTE: THE NEXT DAY 1 OUTLOOK IS SCHEDULED BY 0100Z
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