Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 510 AM EDT Tue Apr 10 2018 Final Day 1, Day 2 and Day 3 QPF Discussion Valid Apr 10/1200 UTC thru Apr 13/1200 UTC Reference AWIPS Graphics under...Precip Accum - 24hr Day 1 For the 12z qpf issuance: Adjustments were made across FL, based on high resolution guidance and radar and satellite trends. The most recent HRRR and 00z NSSL WRF were used depict the axis of heaviest rain expected over north FL, then it was adjust based on radar trends. Elsewhere, little in the way of change was made. ...West Coast/Rockies... A short wave just off the Pacific Northwest coast early on Day 1 crosses Vancouver Island, as a weaker short wave tracks from northern CA to the Northern Rockies. Each short wave taps Pacific moisture for locally heavy qpf amounts. Another short wave approaches the Pacific Northwest late in the period. For the most part, there was good model agreement with the timing of the short waves, so the WPC QPF was based on a blend of the 00z GFS/12z ECMWF. Some 00z HREF mean was utilized to better delineate the higher terrain qpf maxima. Ahead of the first short wave (at the base of a long wave trough off the Pacific Northwest coast) that tracks from off the OR coast to Vancouver Island during the first part of Day 1, a 30 to 50 knot low level southwest flow transports 0.75/1.00 inch precipitable water air over northern CA into western WA. The upslope flow, combined with the lift associated with the short wave, is expected to produce a stripe of 0.75 to 1.25 inches of qpf along the higher terrain of the OR/WA Cascades, mainly before 11/00z. Locally higher qpf amounts (nearing 2.00 inches over the Olympic Range in WA) are possible, as the upslope flow is most parallel to the terrain in western WA. A piece of short wave energy streaks from northern CA across the Northern Rockies between 11/00z and 11/12z. Some of the Pacific moisture plume survives into the Northern Rockies, with precipitable water values between 0.50/0.75 inches (which approaches two standard deviations above the mean). Model soundings showed marginal instability (mainly in the form of MUCAPE values between 100/250 J/KG) between 10/18z and 11/04z, which could add a convective flavor to the precipitation. The combination of moisture, instability and upslope flow is expected to produce local 0.50/0.75 inch qpf amounts over the central Bitterroot Range in ID and the Grand Tetons and Wind River Ranges in WY. Lowering snow levels should mitigate the threat for flash flooding. though low level heavy rainfall and snowmelt could result in some runoff issues. Another short wave in the flow approaches the Pacific Northwest late in the period, with most of the qpf with this piece energy occurring near or after 11/12z. ...Great Lakes/Northeast... Short wave energy tracks from the Upper Great Lakes across northern New England during Day 1, and the short wave pushes along a weak surface reflection. Lift associated with the short wave is expected to produce mainly light qpf, as the airmass in place is relatively dry. For the most part, there was good model agreement with the timing of the short wave, though there were some differences in the qpf coverage across New England. In an attempt to mitigate some of the differences, the WPC QPF was based mainly on a blend of the 00z NAM/00z GFS/12z ECMWF. The short wave tracking from the Upper Great Lakes reaches northern New England during Day 1. The airmass in place is very dry (with 00z soundings showing precipitable water values under 0.15 inches), so qpf amounts associated with this feature are expected to remain below 0.15 inches across MI/northern WI into the UP of MI. Further east, moisture from an offshore low could become funneled into northern New England ahead of the short wave, and the moisture in an upslope flow over VT/NH/western ME could produce local 0.15 inch qpf amounts here as well. ...Southeast/FL... Deep moisture and instability along and south of a quasi stationary boundary extending across north central FL will feed convection that produces heavy to locally excessive rainfall during Day 1. While there is some model consistency on the placement of the front (at least through 11/00z), there are still some significant differences in the placement of the maximum qpf, due mainly to the presence of mesoscale boundary into central FL. In an attempt to mitigate some of the qpf placement differences, the WPC QPF was based on a blend of the 00z WRF ARW and the 00z HREF mean, then adjusted for radar trends in the near term portion of the forecast. Surface based convection continues to fire along the west central coast of FL early this morning, ahead of a wavy front extending from northwest to southeast across central FL. East of the front, the low levels have cooled and stabilized, as cold pool influences have pushed the front south of KMLB. In the near term, the surface based convection is expected to drop slowly south (through at least 10/15z) as cooler/more stable air to the north pushes south. This should result in a west to east line of at least broken convection from north of the KTPA area to south of KDAB, and mirrors the overall presentation during this period of the 00z HREF. After what looks like a break in the convection through the mid morning hours (based mainly on radar and satellite trends), the forecast problem becomes whether there will be sufficient instability left in place for allow surface based convection to fire once again. This would occur after 10/15z or so, and there is quite a bit of model spread as to where (and even whether) this occurs. For now, a blend of the more reliable high resolutions models (including the 00z HREF mean and the 00z WRF ARW) suggest that there should be enough instability to refire convection over central FL. However, whether the convection will produce locally excessive rainfall remains problematic. The WPC QPF followed the idea of convection dropping south with time, but slowly as the front through 11/00z remains quasi stationary. North of the front this morning, elevated convection remains possible as MUCAPE values hover between 500-1000 J/KG through 10/18z. Rainfall amounts here should be lighter, as the isentropic lift north of the front is not that strong. After 11/00z, as a short wave approaches from AL, surface low pressure is expected to organize off the Mid Atlantic coast. During this time, divergence in the right rear quadrant of a 130 knot jet streak stretching from the Mid Atlantic into southern New England is expected to help the convection become more organized. The threat for flash flooding south wane during this period, and become negligible as the short wave pushes the surface front off the southeast FL coast by 11/12z. Days 2/3 ...West Coast...Intermountain West to the Upper Midwest... A potent shortwave trough swings around the Gulf of Alaska low digging an upper trough down the west coast through Wednesday night. This trough will close into a low over the central Rockies Thursday night and be a notable weather maker this weekend into next week. Notable Day 2 (12Z Wed-12Z Thu) precip is associated with the cold front that pushes ashore north from northern CA. 0.5 to 0.75 inch PW spreads into the coast ahead of the front which allow locally moderate precip particularly for the coast ranges north from the Mendocino County CA Wednesday into Wednesday night. The QPF max looks to be around two inches near the coast at the CA/OR border with 1.0 to 1.5 inch from the Sierra Nevada up the Cascades and also higher elevations of northeast OR/northern ID. Low level low development that pushes across the northern Rockies provides the main focus for precip for Day 3 (12Z Thu-12Z Fri). An upslope easterly flow develops over MT north of the low Thursday. A deformation zone then sets up north of the low as it pushes onto the plains with swaths of moderate precip over SD extending east through the rest of the night. Good agreement in precip placement and magnitude warranted a 00Z general model blend favoring the GFS/ECMWF for Day 2 and a general model blend favoring the 00Z ECMWF was conducted for Day 3. ...Northern Plains across Great Lakes to New England... A shortwave trough rounds the western CONUS ridge and shifts east from MT Wednesday, traversing the Great Lakes Wednesday night and Thursday, reaching Maine Thursday night. Western Gulf moisture wraps around the surface low Wednesday night and allows moderate precip over WI/MI. The deformation zone then shifts from Lake Huron across southeastern Canada, only to cross northern Maine Thursday night. The 00Z ECMWF shifted a bit south across the Great Lakes which is in better agreement with the GFS/NAM/UKMET than the 12Z ECMWF. ...Southern Plains... A deep fetch of western Gulf moisture rides a 50 kt southerly jet across TX and OK ahead of a developing surface low over the central plains Thursday night. 1.75 to 2 inch PW is two standard deviations above normal into MO. Heavy thunderstorms may hold off until diurnal heating Friday, but the ingredients will be coming together late Thursday night. Hayes/Jackson Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml