Issued by NWS Cheyenne, WY
000 FXUS65 KCYS 210334 AFDCYS AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE CHEYENNE WY 934 PM MDT SAT SEP 20 2008 .UPDATE...UPPER LOW SPINNING INTO CENTRAL ID CURRENTLY. SEVERAL SHORTWAVE TROUGHS ROTATING AROUND IT HAVE TRIGGERED BANDS OF SHOWERS/TSTMS WHICH HAVE BEEN MOVING ACROSS THE CWA THIS EVENING. WHILE MOST OF THE TSTMS HAVE REMAINED WEAK...A STRONGER ONE IS CURRENTLY ABOUT 40 ENE OF KCYS. LIGHTNING DATA SUGGESTS THAT DESPITE TIME OF YEAR...THERE SHOULD BE ENOUGH DYNAMICS TO KEEP AT LEAST A COUPLE OF TSTMS GOING MOST OF THE NIGHT SO ADDED THESE TO THE SHOWERS THAT PREVIOUS FORECAST HAD OVERNIGHT. JLH && .PREV DISCUSSION... /ISSUED 210 PM MDT SAT SEP 20 2008/ SHORT TERM...TONIGHT THROUGH MONDAY... LATEST U/A ANALYSIS HAS WEAK WARM RIDGE MOVING EAST GIVING THE CWA SW FLOW ALOFT. LOW PRESSURE INDICATED AT BOTH 7/5H OVER OREGON WITH MULTIPLE SHORT WAVES ROTATING AROUND THE LOW AT THE SURFACE LOW CENTERED IN THE PANHANDLE OF IDAHO WITH A WARM FRONT STRETCHING EAST INTO THE DAKOTAS AND A COLD FRONT SOUTHWARD INTO NEVADA. MODELS ARE ALL SHOWING THIS UPPER SYSTEM OVER OREGON MOVING TO THE NORTHEAST ACROSS MONTANA. THIS SOLUTION WILL RESULT IN THE CWA BEING IN DIFFLUENT FLOW ALOFT AND WILL SHOULD ALLOW FOR ISOLATED TO SCATTERED CONVECTION TO DEVELOP. LOOKS LIKE THE BEST CHANCE OF PRECIPITATION WILL BE ON SUNDAY. SUNDAY SHOULD SEE ANOTHER DAY WITH ABOVE NORMAL HIGHS THEN MONDAY SEE COOLING FROM WEST TO EAST. LONG TERM...MONDAY NIGHT THROUGH FRIDAY... RATHER PROGRESSIVE SYNOPTIC PATTERN EARLY IN THE PERIOD WILL TURN MUCH MORE ZONAL ACROSS THE NORTHWEST U.S. AND THE NORTHERN ROCKY MOUNTAINS BY WEEKS END. INITIAL SHORTWAVE...LOCATED NEAR 45N/150W CURRENTLY...WILL BE MOVING THROUGH THE CWA MONDAY NIGHT. SOME SLIGHT TIMING DISCREPANCIES EXIST BETWEEN THE ECMWF/GFS SOLUTIONS...WITH THE GFS 6 TO 12 HOURS QUICKER. AT ANY RATE...HAVE GENERALLY TRENDED PRECIPITATION CHANCES DOWN LATE MONDAY AS FLOW BEGINS TO TURN ZONAL ABOVE THE DEVELOPING FOUR CORNERS HIGH PRESSURE. EXPECT DRY CONDITIONS TO PREVAIL TUESDAY THROUGH FRIDAY AS MUCH OF THE ENERGY BECOMES SHUNTED NORTH. THE EXACT STRENGTH OF THIS AREA OF HIGH PRESSURE WILL DICTATE JUST HOW WARM WE GET THROUGH THE WEEK. HAVE GENERALLY TRENDED TOWARDS OR JUST SLIGHTLY COOLER THAN THE ENSEMBLE MEANS...WHICH WILL STILL BE 5 TO 8 DEGREES ABOVE CLIMATOLOGICAL NORMALS. AVIATION...OUTSIDE OF SOME MVFR CONDITIONS ASSOCIATED WITH ISOLATED SHOWERS OR THUNDERSTORMS THIS EVENING ALL LOCATIONS ARE EXPECTED TO SEE VFR CONDITIONS. && .CYS WATCHES/WARNINGS/ADVISORIES... WY...NONE. NE...NONE. && $$ SHORT TERM/AVIATION...GRIFFITH LONG TERM...HAHN
