Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 615 AM EDT Fri May 25 2018 Final Day 1, Day 2 and Day 3 QPF Discussion Valid May 25/1200 UTC thru May 28/1200 UTC Reference AWIPS Graphics under...Precip Accum - 24hr Day 1 ...Overview... The flow pattern is not too exciting except for a closed low entering the west coast and a broad area of anomalously low heights in the Gulf of Mexico / northern Caribbean where the National Hurricane Center places high probabilities of subtropical or tropical depression development in the near future. A shortwave digging into the back side of the Gulf trough early this morning will help induce inverted troughing and enhanced southerly flow of very moist air downstream over south Florida today. Elsewhere northern stream waves will promote a few mesoscale thunderstorm clusters over the upper Midwest / Great Lakes and New England. Diurnal convection should again become fairly widespread in coverage over the southern tier, and the low along the west coast will yield organized rainfall, especially northern California and southern to eastern Oregon. WPC QPF was derived from a near 50/50 blend of our in-house ensemble and the HREF blended mean. Six-hour QPF probabilities from the GEFS were also helpful. ...Western U.S... Models are very generous with rainfall in the difluent region north to northeast of the closed low center. Although the low is forecast to open up late in the Day 1 period there should be a lengthy duration of favorable ascent profiles and southeasterly low level inflow from northern California into Oregon, and catching part of northwest Nevada. The event may begin with some convective enhancement and then transition to longer duration stratiform rainfall behind the surface boundary. An environment more supportive of deep convection and/or more repetitive convection should exist farther east, from southeast Oregon into parts of northern Nevada and much of Idaho. Instability drops off with eastward extent through NV/UT given drier air in that region, but thunderstorms should be able to make some eastward progress before drying out. WPC favored the NAM CONUS NEST for its more expansive coverage, but perhaps the HREF mean for more toned down amounts. Still, areal average rainfall greater than an inch is forecast in parts of CA/OR. The odds of training cells appears lower today, but hourly rain rates will likely exceed a half an inch locally during the stronger individual thunderstorms. ...Florida... With a shortwave trough dropping in to strengthen the broad upper trough over this region, the models develop an inverted trough axis over south Florida along with upwind convergence in the strengthening and very moist southeasterly flow coming off the Caribbean. There is an increased model signal for widespread 1 to 2 inch areal average rainfall, and the potential for locally more intense amounts as the pattern remains locked in, allowing repeated tropical convection into the Keys and parts of south Florida. ...Central / Eastern U.S... The small vort max that has aided in rainfall production over the southeast will lift a bit and begin to shear toward the Carolinas. On the broader scale, the response to progression in the northern stream will start to draw higher octane air back into the eastern Tennessee and upper Ohio Valleys. The coverage of measurable precipitation, therefore, is expected to increase or at least expand toward the north compared to recent days. The pattern overall still lacks a lot of focus, however, with very weak winds in much of the column over the eastern and southern states. Some stronger shear profiles and forcing could lead to more organization over the upper Midwest, and early day warm advection followed by strong heating will again allow for a few clusters of pseudo-organized convection in the lower Missouri Valley and southern Plains. WPC favored many of the details in the NAM CONUS Nest, although the lack of synoptic control on the forecast does give us less confidence, and for QPF we mainly relied on a blend of low-res and high-res ensemble output, using our in-house ensemble and the HREF blended mean. The result was areal average rainfall of a quarter inch or more, but not exceeding one inch, over broad areas from the upper peninsula of Michigan to Oklahoma and over across the southern states. Days 2/3... ...Southeast and Florida Peninsula... Heavy rainfall is likely this weekend across a wide swath of the Southeast and Florida. Much of this will be related to the developing low in the Gulf of Mexico that may become a subtropical or tropical cyclone. For the latest on the tropical development, refer to National Hurricane Center forecasts. The primary uncertainties with the QPF are related to the timing and track of the low, as that will have a large influence on the eventual distribution of rainfall. The preference was to follow a model blend closest to the 00Z ECMWF, with some 00Z CMC incorporated as well. The CMC is further east, but represents a reasonable possibility if the surface low gets pulled east by large, sheared convective clusters. The timing of these models (and most available deterministic models and ensemble members) generally keeps the surface low offshore of the central Gulf Coast by the end of the Day 3 period (12Z Monday). Therefore, most of the Day 2-3 QPF will be from scattered convection ahead of the advancing low, or outer convective bands in the warm conveyor belt to the east. Models all generally show an evolution to a more subtropical configuration of the low by the Day 3 period, with a prominent mid-level dry slot wrapping around the south, and eventually east, side of the circulation. This may bifurcate the heavy rainfall, with one concentrated area very close to the low center, and another to the east in the undisturbed PW plume associated with the warm conveyor belt. The largest differences in WPC QPF relative to the NBM are a decrease in QPF associated with the initial northward push of rain on the north and northeast side of the circulation (indicating a slower progression), and an increase in QPF over the Florida Peninsula given the possibility of a slightly more eastward track. WPC QPF also was higher than the NBM over the coastal Carolinas on Day 3 given a strong signal for the right entrance region of a 60 knot upper level jet interacting with the northward-advancing tropical moisture plume. Overall, though, WPC QPF remained fairly consistent with the previous forecast. A Slight Risk of Excessive Rainfall was introduced on Day 2 in the southern Florida Peninsula as the cyclonic circulation begins to advance into the state with PWs climbing well above 2 inches. A more expansive Slight Risk was introduced on Day 3 for much of the Southeast as the extent of the moisture plume becomes more broad. Refrained from more elevated risk categories at this time given (1) the expectation that the core of the low should still be offshore at the end of the Day 3 period, and (2) uncertainty about the placement of outer rain bands to the east. Most models show a gradual eastward drift to the predominant rain band over Florida, suggesting that the rain band may not lock in over a particular area for an extended period of time. Nevertheless, given the tropical environment with deep, saturated profiles and PWs approaching 2.5 inches, any convective rain bands should be capable of producing significant rain rates and therefore flash flooding. ...Mid-Atlantic... WPC QPF in the Mid Atlantic was largely consistent with the NBM for the Day 2-3 period, although amounts were increased from southeast Pennsylvania into New Jersey on Saturday Night. Several rounds of rain appear likely, one with a front (and broad trough aloft) advancing into the region on Saturday, and the next with a wave developing along the stalling front on Sunday. The environment along and ahead of the front should be fairly moist, with PWs on the order of 1.7 to 2.0 inches in some places. These PWs would be above the 99th percentile for late May from the central Appalachians into the coastal Mid Atlantic. Given the anomalously high moisture values and unidirectional flow (that may promote training of convection), a marginal risk was introduced on both Day 2 and Day 3. If confidence increases on placement of more organized convection, an upgrade to Slight Risk will be possible in future outlooks. ...Intermountain West, Northern Rockies, and Northern Plains... An upper level low will slowly lift northeast through the Intermountain West in the Day 2-3 period, focusing some scattered convective rainfall through the region and just downstream into the Northern Rockies. In general, did not deviate far from a broad-based model blend and the NBM. Ridging downstream into the Plains should be quite pronounced, with a substantial EML extending well up into the Dakotas. Potential exists for one or more convective clusters to emerge from the higher terrain in Montana and Wyoming and then track east through the Dakotas. Adjusted QPF to fit generally north of model consensus +11C to +12C isotherms at 700mb, and increased the QPF slightly given the expectation that convection may be more focused into organized clusters of thunderstorms on the nose of the thermal ridge. Burke/Lamers Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml