Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 457 PM EDT Fri May 18 2018 Prelim Day 1, Day 2 and Day 3 QPF Discussion Valid May 19/0000 UTC thru May 22/0000 UTC Reference AWIPS Graphics under...Precip Accum - 24hr Day 1... ...Mid Atlantic and Southeast... Another wet day with locally excessive rainfall expected across the area. The mid and upper levels remain broadly diffluent ahead of the large scale trough, with multiple weak embedded shortwaves continuing to progress northward in the southerly flow. PWATs remain in the well above normal to near climatological record range, supportive of efficient rainfall processes. A nearly stationary west to east boundary will remain draped across central VA through much of Friday. This boundary will once again act as a focus for backbuilding/training convection, with continued 850 mb moisture transport into the boundary. Modest instability remains along this front (mixed layer CAPES 500-1000 j/kg), allowing for an uptick in rainfall rate potential this afternoon/evening -- particularly across southern-central VA. North of the front, rainfall will be more stratiform in nature, resulting in generally lower rates. South of the front, convection will generally be less organized, but still capable of briefly intense rates. Thus the best focus for heaviest rains and an increasing flash flood threat will be along/near the front, and into the northwest NC and southwest VA terrain, where differential heating across the higher terrain and some upslope component to the flow may enhance convective coverage and rainfall magnitudes. Overall a good consensus amongst the 12z HREF members to focus higher amounts near this front, with high probabilities of exceeding 3" and low to moderate probabilities (20 to 40%) of localized 5"+ amounts. Overall appears like a very similar setup as yesterday. Given this additional rainfall potential is over increasingly saturated grounds, a more widespread and significant flash flood threat appears to exist today. Thus have opted to go with a MODERATE risk of excessive rainfall across portions of southern and central VA into southern MD. North of the front, rainfall will generally be more stratiform in nature, keeping rates lower. However could still see some localized heavier rates, especially tonight as the whole system lifts northward. This northward shift should dissipate the stationary boundary over VA, and thus think by this time we should see an end to the better backbuilding/training setup over southern/central VA. However should still see showers and embedded thunderstorms shifting north across the Mid Atlantic, which may focus into a narrower north/south axis. Instability should be decreased by this point, tempering rates, although localized heavier amounts are possible where any south to north training is able to occur. Given the lower FFG here, will maintain a Slight risk across northern VA into portions of MD and DE. Will also will need to watch portions of eastern NC/SC later tonight and on Saturday. Looks like a mid/upper level shear axis (vort lobe) and attendant low-level convergence axis will try to set up here, with a long fetch of moisture streaming north out of the Caribbean and over the Gulf Stream (where current and forecast 500-300 mb layer PW values are expected to be around 0.25"). Thus possible we see south to north streamers of convection through the day, capable of producing very heavy rainfall rates. The models, particularly the CAMs, have backed off in terms of the timing (now mainly after 09Z Sat). FFG is higher here, and not certain we will see convection organize enough to pose a more widespread flooding threat. The 12z HREF does indeed shower some higher probabilities of exceeding 3" and 5" here, though mainly after 12z Sat. Thus see no strong reason to remove the Slight risk that was in place, and thus just adjusted it a bit to better fit the newer guidance. ...Central Plains... Another round of storms will fire across CO/WY late this afternoon into the evening coinciding with the peak diurnal destabilization and reasonably steep 700-500 mb lapse rates. Will also be a more favorable synoptic pattern compared to yesterday, with the broad mid/upper level trough and embedded shortwaves moving closer. PWAT values will remain well above normal to near climatological record values as well. Anticipate a complex evolution to convection across this area with multiple factors at play. Will have a very strong gradient in place over the western Plains, with moderate/high instability forecast. An 850 mb low over southern CO will strengthen southerly and easterly flow ahead of the instability gradient feeding into convection. Will also heave a northern stream cold front progressing south with time across the area. In general anticipate we will see widespread convective development, with some chaotic storm motions resulting in areas of training and repeat convection. Will likely end up seeing all this convection result in a well defined MCV taking shape as well, adding even more complexity to the evolution. Thus for QPF, WPC generally followed a blend of the 12z HREF members. Tough to pin down exactly where the heaviest amounts will fall, but think at least some flash flood risk will likely evolve with time into tonight from northeast CO into southeast WY, western and central NE, and western KS. Storms further south into OK/TX should generally be less organized and more progressive being displaced further form the better deep layer forcing and better 850 mb moisture convergence. ...Mid MS Valley into the OH/TN Valley... Scattered to widespread convection is likely across this region today underneath the mid level shear axis and embedded shortwave features. This setup supports slow moving convection, and periodic cell mergers, and with PWATs running well above normal, locally excessive rainfall will be possible. WPC generally followed close to the 12z HREF. As was the case in previous days underneath the shear axis, amounts will likely be higher than the global models suggest, and thus not surprising that this blend results in a pretty significant increase in amounts from our previous forecast. Days 2/3... ...Central Plains/Mid and Upper MS Valley to the OH Valley... A low-mid level wave over CO/KS Saturday evening shifts east-northeast to IA through Sunday in the wake of an upper trough shifting east across Ontario. Gulf moisture wraps around the low with PW of 1.5 inches which is 1.5 standard deviations above normal over eastern NE and IA where heavy rain is expected in advance of the surface low. The 12Z ECMWF came in better alignment with the preferred 12Z GFS/UKMET solution with a more amplified Canadian trough and farther south low (earlier runs gave a fair amount of rainfall to SD which is no longer expected). The Day 2 Slight risk for excessive rain was maintained and shifted a bit southeast given the consensus of 12Z guidance. The low shifts east through Sunday night with the heavy rain/deformation zone focus over eastern IA/northern IL. The Marginal risk for excessive rainfall was shifted northeast a bit per 12Z consensus which now includes Chicagoland. ...FL... Deep moisture within confluent flow west of a Bermuda high will support bands of showers over eastern FL on day 2 and then shifting west on day 3 to the western half of the FL peninsula. Showers continue on day 3 in southern peninsula to the keys as the moisture stream originates from the Caribbean and crosses the FL straights into the area with 2-2.25 inch pw values. The combination of moisture and instability should result in moderate to heavy rainfall, especially across the southern peninsula of FL and into eastern NC. WPC QPF was based on the 12Z GFS/ECMWF which is in good agreement for Day 2 and convective elements will determine the positioning farther north up the peninsula for Day 3, so a general model blend was used for QPF then. Marginal risks for excessive rainfall over the southern FL peninsula were maintained for both days 2 and 3. ...Carolinas/Central Appalachians/Mid Atlantic/Northeast... Deep moisture streaming northward from the Southeast states crosses the OH Valley into the northeast with a pair of 700 mb waves forecast to produce lift and resultant showers across the lower Great Lakes/New York/New England as the stationary front shifts north Saturday night. Once the wave moves off the coast on Sun night, the coverage/amounts of rainfall taper in NY/New England. A day 2 (12Z Sat-12Z Sun) Marginal Risk for excessive rainfall as raised for an arc from the central Mid-Atlantic coast to NYC to the central Appalachians for the streaming tropical moisture up the coast and the upper low lifting remnant moisture over western slopes of the central Appalachians. The ARW 2 hi-res model was consulted for the Saturday night QPF, otherwise a 12Z GFS/ECMWF/UKMET blend was used. The slight risk was continued for southern portions of eastern NC Day 2 as agreement increases for a narrow persistent band of heavy rain Saturday. Hi-res guidance agrees with the placement or shifts it a bit west like the 12Z NAM. Therefore the Slight risk area was expanded west a bit. ...Pacific Northwest/Northern Rockies/Great Basin... A long wave trough will slowly amplify this weekend as it crosses OR before closing into a low over southern CA Monday. The focus of rainfall develops with upper divergence maxima at the base on the trough crossing northwest CA to northern NV late Saturday and continues in northern NV to OR as additional upper divergence maxima support widespread showers that spread down to the Sierra Nevada. Good agreement in the 12Z suite of guidance allowed a general model blend for QPF. ...West Texas/eastern NM... East of the developing western US trough is confluent flow over Eastern NM and Texas with southeast flow bringing gulf moisture into NM. A Marginal Risk for excessive rainfall was raised for north of the Big Bed to the NM border for 1.5 inch PW Sunday night which is 2 standard deviations above normal with high CAPE and low shear which should keep storm motion slow. Hurley/Jackson Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml