286 FXUS66 KPDT 231803 AAA AFDPDT AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE PENDLETON OR 1058 AM PDT THU MAY 23 2013 UPDATED THE AVIATION DISCUSSION .SHORT TERM...TODAY AND TONIGHT...A LARGE UPPER LEVEL LOW PRESSURE SYSTEM WILL REMAIN OVER THE REGION WITH UNUSUALLY COLD AIR ALOFT ASSOCIATED WITH IT. THIS COLD AIR ALOFT COMBINED WITH THE SURFACE HEATING WILL CREATE AN UNSTABLE ATMOSPHERE DURING THE AFTERNOON AND EVENING RESULTING IN SHOWERS AND SOME ISOLATED THUNDERSTORMS. THIS HAS BEEN THE PATTERN THE PAST COUPLE OF DAYS AND WILL BE THE CASE AGAIN TODAY. THE INSTABILITY SHOWERS WILL DECREASE AROUND SUNSET. MODELS ARE ALSO SUGGESTING AN AREA OF MORE STEADY PRECIPITATION WRAPPING AROUND THE LOW PRESSURE INTO CENTRAL OREGON LATE TODAY AND OVERNIGHT. THIS COULD PROVIDE SOME OVERNIGHT STEADY PRECIPITATION TO THIS PORTION OF THE FORECAST AREA NOT RELIANT ON THE UNSTABLE CONDITIONS DISCUSSED PREVIOUSLY. TEMPERATURES WILL ALSO REMAIN COOL AND BELOW NORMAL FOR THIS TIME OF YEAR WITH SNOW LEVELS BETWEEN 4000 AND 5000 FEET. CONVECTIVE BURSTS OF SNOW ASSOCIATED WITH TODAYS CONVECTION ARE POSSIBLE WHICH COULD RESULT IN SNOW LEVELS BELOW 4000 FEET UNDER CONVECTIVE CELLS. NOT EXPECTING THE NEED FOR ANY FREEZE WARNINGS MAINLY DUE TO ANTICIPATED CLOUD COVER OVERNIGHT. HOWEVER...IF SOME PROLONGED CLEARING TAKES PLACE OVERNIGHT IT IS POSSIBLE FOR TEMPERATURES TO SPIKE DOWN RAPIDLY UNDER THIS WEATHER PATTERN. .AVIATION...A LOW PRESSURE SYSTEM IS FORECAST TO MOVE INLAND AND INCREASE CLOUDS AND SHOWERS ACROSS THE AREA INTO TONIGHT. MVFR CEILINGS WILL CONTINUE AT TAF SITE KDLS IN AND NEAR SHOWERS. OTHERWISE DETERIORATING CEILINGS CAN BE EXPECTED AT TAF SITES KYKM AND KRDM THAT WILL LOWER TO AREAS OF MVFR CONDITIONS AFTER 22Z. AFTER 02Z THE REMAINDER OF THE TAF SITES SHOULD EXPERIENCE MVFR CEILINGS IN SHOWERS INTO FRIDAY MORNING. && .PREV DISCUSSION... /ISSUED 155 AM PDT THU MAY 23 2013/ SHORT TERM...TODAY THROUGH FRIDAY NIGHT...THE MODELS ARE IN VERY GOOD AGREEMENT WITH THE POSITION AND STRENGTH OF A LARGE UPPER LOW PRESSURE SYSTEM OVER THE REGION THAT HAS BEEN PROVIDING SIGNIFICANT PRECIPITATION. THE FORECAST MODELS REMAIN IN GOOD AGREEMENT THROUGH TONIGHT AND THEN THEY BEGIN TO DIVERGE FROM EACH OTHER ON FRIDAY. IN ANY EVENT THEY ALL KEEP AN UPPER LOW OR AN UPPER TROUGH OVER THE PACIFIC NORTHWEST THROUGH THE SHORT TERM PERIOD. THIS WILL KEEP A COOL AND UNSETTLED WEATHER REGIME OVER THE FORECAST AREA FOR THE SHORT TERM FORECAST PERIOD. TODAY THE CENTER OF THE UPPER LOW IS EXPECTED TO PIVOT BACK AROUND TO THE WEST AND THEN DROP SOUTHWARD WEST OF THE CASCADES TONIGHT AND THEN EASTWARD AGAIN. THIS WILL BRING A SLUG OF MOISTURE BACK WESTWARD AND THEN SOUTHWARD AND EASTWARD ACROSS SOUTHERN PORTIONS OF THE FORECAST AREA. THE SOUTHERN FORECAST AREA HAS SEEN THE LEAST AMOUNT OF PRECIPITATION SO FAR WITH THIS SYSTEM. THERE IS A POTENTIAL THAT THESE AREAS (CENTRAL OREGON AND THE OCHOCO-JOHN DAY HIGHLANDS) COULD SEE MORE SIGNIFICANT PRECIPITATION BEGINNING THIS AFTERNOON AND CONTINUING THROUGH TONIGHT INTO FRIDAY MORNING. THE ATMOSPHERE IS FORECAST TO BECOME UNSTABLE THROUGH MUCH OF THE FORECAST AREA FROM THE EASTERN MOUNTAINS ACROSS THROUGH CENTRAL OREGON AND UP ALONG THE CASCADE EAST SLOPES INTO SOUTH CENTRAL AND CENTRAL WASHINGTON. HAVE DECIDED TO INCLUDE ISOLATED THUNDERSTORMS IN MOST AREAS OF THE CWA FOR THIS AFTERNOON AND THIS EVENING EXCEPT THE LOWER COLUMBIA BASIN AND THE BLUE MOUNTAIN FOOTHILLS. THE ATMOSPHERE BECOMES UNSTABLE AGAIN FRIDAY AFTERNOON AND ALTHOUGH NOT AS WIDESPREAD HAVE ALSO INCLUDED A SLIGHT CHANCE OF THUNDERSTORMS AGAIN IN MUCH OF THE SAME AREAS FRIDAY AFTERNOON AND EVENING. THE UPPER LOW BECOMES ELONGATED FROM SOUTHWEST TO NORTHEAST LATE FRIDAY NIGHT AND EARLY SATURDAY RESULTING IN LESS ORGANIZATION OF THE OVERALL SYSTEM. THE RESULT WILL BE DECREASING COVERAGE OF PRECIPITATION ACROSS THE FORECAST AREA FRIDAY NIGHT. TEMPERATURES WILL REMAIN COOLER THAN NORMAL OF ABOUT 8 TO 12 DEGREES ACROSS THE AREA...ESPECIALLY DAYTIME HIGHS. FREEZE WARNINGS ARE IN EFFECT THIS MORNING FOR THE SOUTHERN BLUE MOUNTAIN FOOTHILLS AND THE GRANDE RONDE VALLEY. THERE MAY BE SOME SNOW IN THE SIMCOE HIGHLANDS AND IN THE UPPER CASCADE EAST SLOPES BUT AT THIS TIME SNOW AMOUNTS WILL BE UNDER ADVISORY CRITERIA IN THESE AREAS. 88 LONG TERM...SATURDAY THROUGH THURSDAY...THE UPPER LOW OVER THE AREA WILL LIFT INTO SOUTHERN CANADA OVER THE WEEKEND WITH SLIGHTLY LOWER CHANCES FOR SHOWERS. HOWEVER...ANOTHER LOW WILL DROP SEWD INTO THE REGION FOR MUCH OF NEXT WEEK WITH INCREASING CHANCES FOR SHOWERS...WITH THE BEST CHANCE LOOKING TO BE LATE MONDAY THROUGH TUESDAY. THERE COULD BE AN ISOLATED THUNDERSTORM OR TWO THROUGH THE PERIOD BUT AS OF NOW IT DOES NOT LOOK LIKE THE ACTIVITY WILL BE TOO SIGNIFICANT. TEMPERATURES WILL BE AROUND NORMAL FOR THIS TIME OF YEAR WHICH EQUATES TO 70S FOR HIGHS LOWER ELEVATIONS AND 50S AND 60S MOUNTAINS. && .PRELIMINARY POINT TEMPS/POPS... PDT 61 42 62 41 / 30 30 50 20 ALW 64 42 63 44 / 30 30 40 20 PSC 67 43 67 42 / 20 20 40 20 YKM 60 42 63 43 / 20 20 40 30 HRI 66 44 66 42 / 20 20 50 20 ELN 58 38 60 42 / 30 30 40 30 RDM 56 38 56 32 / 30 60 40 20 LGD 58 35 55 36 / 30 40 50 20 GCD 56 34 54 32 / 30 40 50 20 DLS 61 44 64 45 / 20 60 50 20 && .PDT WATCHES/WARNINGS/ADVISORIES... OR...NONE. WA...NONE. && THREAT INDEX TODAY : GREEN FRIDAY : GREEN SATURDAY : GREEN GREEN: NONE OR LIMITED NEED FOR WATCHES, WARNINGS, OR ADVISORIES. YELLOW: A FEW WATCHES, WARNINGS, OR ADVISORIES POSSIBLE OR IN EFFECT. RED: NUMEROUS WATCHES, WARNINGS, OR ADVISORIES POSSIBLE OR IN EFFECT. FOR ADDITIONAL WEATHER INFORMATION, CHECK OUR WEB SITE AT... WWW.WEATHER.GOV/PENDLETON $$ 91/97