Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 432 AM EDT Wed Mar 14 2018 Final Day 1, Day 2 and Day 3 QPF Discussion Valid Mar 14/1200 UTC thru Mar 17/1200 UTC Reference AWIPS Graphics under...Precip Accum - 24hr Day 1 ...Northeast... Snow continues this morning across much of ME to the west of the strong offshore low. The low will weaken today as it moves west into southern Canada...however snow showers will continue underneath the closed mid/upper level low over much of central and northern New England. Northwesterly flow on the backside of the low will result in orographic enhancement to the snow over the favored terrain of upstate NY and VT/NH...with lake enhancement also expected off of lake Ontario and Erie. Overall models were in very good agreement through the day 1 period. WPC QPF was thus based on a multi model blend. ...West... Longwave troughing building into the west will continue the showery pattern through the day 1 period. The better PWATs and moisture transport will have shifted south and east of the west coast...thus not anticipating rainfall rates or totals to be that impressive. However with the trough axis moving ashore, showery conditions will continue...with a well defined wave on the southern periphery of this trough helping increase shower coverage over central CA. Overall model trends have been for less QPF today across the Sierras...with additional amounts generally forecast between 0.5"-1". Did increase totals some across the southern CA mountains though...with increased onshore upslope flow anticipated today and slightly better PWATS there. Looking at some 1" amounts over portions of the Transverse and Peninsular Ranges. Showers will overspread the Great Basin as well though the day. Falling heights aloft and good upper level divergence ahead of an approaching jet will provide a favorable environment for increasing shower coverage. Steepening mid level lapse rates should also allow for the development of some weak instability today. Thus would expect scattered convection to develop...primarily from eastern NV into UT. PWAT values do increase to around 0.5" over UT as well...which is about as high as they can get this time of year. Thus would anticipate some brief heavy rainfall rates to accompany any showers/storms that do intensify. However storm motions will be very quick...thus not thinking localized amounts will get much above 0.5" in an hour. Areal averaged rainfall generally expected to between 0.25"-0.5" across portions of NV/UT...with amounts approaching 1" over northern UT where orographic enhancement is expected. Models were close enough to allow for a multi model blend for QPF over the west...with more emphasis given to the high res CAMs. Days 2/3... WPC QPF was based on a blend of the half operational (00Z ECMWF/GFS/UKMET) and half ensemble (00Z local bias-corrected ensemble mean and the 05Z NBM). ...CA/OR... A deep upper low will shift south down the OR coast Thursday before shifting inland across far northern CA Friday. PW advecting into the northern CA coast on Thursday will be around 0.5" which is not anomalous, but the presence of instability under the cold core low (100 to 200 j/kg) will allow locally heavy showers/possible thunder. Maintained the marginal excessive rain risk for the far northern CA coast to Brookings OR. The marginal risk was removed for the Sacramento valley due to low snow elevations and lack of anomalous PW. Orographic lift will lead to heavy precip in the higher coast ranges and the CA Cascades/northern Sierra Nevada where areal average liquid equivalent is 1-2 inches. ...Central Plains/Mid-South... An upper trough will shift east-northeast from southern CA Thursday, cross southern CO Thursday night, and reach NE Friday. Surface low pressure ahead of this shortwave will develop over western KS Thursday night and shift east to MO through Friday night. The combination of low-level moisture from the Gulf of Mexico and mid-to upper-level Pacific moisture from off southwest Mexico is expected to saturate the atmosphere region-wide. An easterly component north of the low looks to make a max precip area (areal average up to one inch rainfall) Thursday night across eastern WY where upslope and upper level forcing combine to enhance precip. Pockets of instability (500-1000 J/kg) near the WY/CO/NE border Thursday afternoon and another pocket moving across KS from Thursday night through Friday which should aid precipitation production. A narrow swath of precip is expected east/north of the developing surface low where southeasterly 40kt flow will bring 0.75in PW air. Low confidence persists on the location of this swath...though half to three quarter inch areal average precip is likely across this portion of the storm. ...Western Gulf coast... Return moisture flowing over a slowing boundary near the LA and Upper TX coasts should lead to showers and thunderstorms Friday as instability builds within the warm sector. Broad 850 hPa inflow of 25-40 kt with PW up to 1.5 inches (1.5 standard deviations above normal) combined with MU CAPE values ascending towards 2500 J/kg and 35-50 kts of 0-6 km bulk shear will attempt to organize convection into short bands as the flow through the troposphere is fairly unidirectional out of the west-southwest. Hourly rain totals up to 1.75" should be possible within this environment. QPF guidance continues to fluctuate in this area with the 00Z GFS the most robust with precip for Day 3. Chenard/Jackson Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml