Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 459 AM EDT Sat May 26 2018 Prelim Day 1, Day 2 and Day 3 QPF Discussion Valid May 26/1200 UTC thru May 29/1200 UTC Reference AWIPS Graphics under...Precip Accum - 24hr Day 1 ...Tropics / Gulf... Sub-tropical storm Alberto is expected to become more organized and strengthen. Most of the heavy precipitation will affect the open waters of the Gulf, but also Cuba through Saturday night. Some heavy rain will fall, however, over the Florida Keys and south Florida. Alberto is embedded with what is still a weakly baroclinic, broad scale trough, and the warm conveyor belt / southerly flow is forecast to migrate out from the center toward Florida by afternoon/evening. This should enhance convergence within the very moist and moistening atmosphere that was in place there. Using the official NHC track for Alberto as a guide, WPC QPF favored the ECMWF, SREF, and continuity in this area. The WRF-ARW2 was one of the more useful hi-res models. The NCEP guidance was generally a little drier than the new 00Z ECMWF, causing us to lower 24-hour totals a tad in the FL Keys. Nevertheless, some tropical downpours are likely, and the potential for focused bands and training of echoes should increase as the system takes shape. The QPF will be revisited and may be massaged upwards for the final 0930Z package. ...Eastern U.S... Sea breezy activity along the Gulf Coast and then a generally supportive mix of May sun angle and plentiful moisture should yield large coverage of measurable rainfall today everywhere east of a Dallas to Green Bay line. Focus for more greatly organized thunderstorms is expected to exist with a shortwave trough over the Ohio Valley, within a confluent flow regime with strong moisture advection over the Mid-Atlantic states, and along a northern stream frontal boundary in New England. WPC QPF was derived primarily from the NAM, NAM CONUS Nest, and WRF-ARW2, as these were more generous with the more moderately forced regimes, especially that in the mid Atlantic. ...Western U.S... The upper low over Nevada will move eastward into areas that quite a bit drier in the low levels, so much of the diurnal thunderstorm activity on Saturday will occur in the post frontal environment back across Nevada and up through the northern Rockies. The models also hint strongly at MCS development over eastern Montana to the north of a developing warm front this evening. WPC QPF favored the WRF-ARW2 and NAM CONUS Nest. Days 2/3... ...Southeast and Florida Peninsula... *** MODERATE RISK of Excessive Rainfall on Monday and Monday Night for far southeast Mississippi, southern Alabama, and the western Florida Panhandle *** The rain associated with Subtropical Storm Alberto will begin spreading into the Southeast region in earnest during the Day 2 period (Sunday and Sunday Night), with the storm's center approaching the central Gulf Coast on the Day 3 period. WPC QPF for the Day 2-3 period includes more than 1 inch over much of the Southeast region from North Carolina and the southern tip of the Appalachians, to near Tuscaloosa, to near New Orleans, and all areas to the southeast. A somewhat bifurcated QPF pattern is anticipated due to a prominent mid-level dry slot expected to wrap around the south and east side of Alberto's center. One QPF maximum is expected near (and perhaps displaced just east) of the track of the center, where a bubble of higher PWATs should remain relatively undisturbed. Another should be situated in the warm conveyor belt from the Florida Peninsula north into the coastal Carolinas. The model that most closely approximated these expectations, as well as the official NHC forecast track, was the 00Z ECMWF. Therefore, WPC QPF was weighted most substantially toward the ECMWF, combined with an internal multi-model ensemble. The QPF distribution was modified slightly to focus heavier QPF more closely to the center overnight, and away from the center during the day as convective instability grows over land areas -- a typical diurnal oscillation with tropical cyclones. WPC QPF is generally higher than the NBM near the track of the center, with the maximum QPF swath displaced east from the NBM position. The NBM appeared to be incorporating more models that have a surface low position west of the NHC track. WPC QPF was also higher than the NBM over the coastal Carolinas. For the Excessive Rainfall Outlooks, the majority of the heavy rainfall threat was addressed with broad Slight Risks over much of the region. This struck a balance between the favorability of the environment with deep tropical moisture, and uncertanties regarding the specific location of mesoscale banding. Two areas of particular concern on the Day 3 outlook (Monday and Monday Night) were considered for a Moderate Risk. (1) Near the track of Alberto on the central Gulf Coast, where the decision was ultimately made to upgrade in collaboration with WFOs MOB and TAE. And (2) along the coastal Carolinas in a region of jet coupling (left exit region of dry slot jet streak; right entrance region of Mid Atlantic jet streak). PWATs there will be above 2 inches, with the potential for a coastal front to aid in focusing convection, but lingering uncertainties about mesoscale details precluded an upgrade for now. The area of heavy rain around the center of Alberto may be relatively small as it approaches the coast, as most models show a relatively small core circulation and limited diameter of heavy QPF. This also shows up in the 3km NAM and HWRF. As such, even small deviations in the track could significantly shift the expected heavy rain and flash flooding impacts near the center. However, there is relatively high confidence in that flash flood threat existing near the track, particularly given the expected slow forward motion (around 7-8 knots). Therefore, the decision was made to go with a Moderate Risk to highlight the expected threat, with the possibility that the exact location may shift with future adjustments to the NHC forecast track. Please refer to the National Hurricane Center forecasts for the latest information on Alberto. ...Mid-Atlantic... A swath of heavy rain is expected across the Mid Atlantic into the Day 2 period, likely as a continuation from the Day 1 period. The combination of a lingering low-level front and thetae gradient, along with the right entrance region of a jet streak, should provide enough focus for over 0.5 inches of QPF from Nrn VA and MD into the Delmarva Peninsula. Given the generally uni-directional flow roughly parallel to the low-level boundary, some possibility of localized heavier totals and flash flooding should exist, and a Marginal Risk was maintained on the Day 2 Excessive Rainfall Outlook. ...Intermountain West, Northern Rockies, and Northern Plains... An upper level low across the Intermountain West and Great Basin should maintain a chance for scattered convection through the Day 2 period, likely concentrated near terrain features, before the low devolves into a trough and kicks into the Plains by Day 3. Generally followed a mixture of the NBM and WPC internal multi-model ensemble in this region. Some of the southward extent of QPF was limited (if not in location, then at least in magnitude), due to the possibility of a fairly robust EML plume, particularly on Day 2. The Slight Risk was maintained on the Day 2 Excessive Rainfall Outlook over southeast Montana and northern Wyoming. Strong ENE low-level inflow should create both convergence and possibly some orographic ascent, and models generally agree on PWATs over 1 inch being advected into the area in this band of low-level inflow. Additionally, upper level divergence should be enhanced by coupled jet structure just northeast of the upper level low. Given the strong synoptic scale forcing and PWATs around the 95th percentile for late May, there seems to be sufficient threat of flash flooding to maintain the Slight Risk. Over an inch of QPF is forecast over southeast Montana. Burke/Lamers Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml