Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 553 PM EST Sat Mar 10 2018 Final Day 1, Day 2 and Day 3 QPF Discussion Valid Mar 11/0000 UTC thru Mar 14/0000 UTC Reference AWIPS Graphics under...Precip Accum - 24hr Day 1... ...Lower MS valley into the TN Valley and Southeast... A heavy rainfall event remains on track late today into early Sunday across portions of AR into northern MS/AL/GA, although some model spread remains (especially with the global guidance) with regards to the exact axis and magnitude of highest totals. The GFS and NAM remain on the northern and more progressive periphery of the model spread, especially when compared to the ECMWF and bevy of high-res CAMs (including the ARW, ARW2, HREF mean). This evening and overnight, we will begin to see better mid/upper level forcing move back into the lower MS valley. In response, we should see an uptick in 850 mb moisture transport and a tightening of the low level west to east stationary boundary. The combination of this increased synoptic support and low level moisture convergence should result in an expansion of convective activity by this evening across AR/MS/AL along/near the aforementioned boundary. Models all seem to be in agreement with this evolution occurring. Considering the quasi-zonal flow aloft and largely parallel frontal/thickness structure at low levels, there is a threat for training convection overnight across this region, at least for brief periods, while overall the hi-res guidance is suggestive of upscale growth and a forward propagating MCS gaining steam over eastern Arkansas by 03-06z. Refer to the latest Excessive Rainfall Discussion or ERD for further details. PWAT values are only marginally above average...and not really at the magnitude you would like to see for efficient heavy rainfall. Although the values approaching 1.2" would still support the potential for some 1-2" an hour rainfall rates. The 850 mb moisture transport is also not as strong as you would normally see for excessively heavy QPF amounts. Instability is not forecast to be that high...although it does look like we will initially see enough surface based instability over AR for development...and enough elevated instability farther east to sustain convection into MS/AL. Thus there will be enough increased mid/upper level forcing and instability present to allow for the development of robust convection by this evening, some of which was already occurring as of 23z. While the magnitude of 850 mb moisture transport is not great, the corridor of enhanced transport is broad. This supports an elongated axis of moisture convergence, which should be supportive of some west to east training and repeat convection. It is this factor that will likely result in a corridor of 2-3" (locally higher) of rainfall and the development of at least some flash flood threat into tonight (despite the marginal precipitable water/instability). Models are in better agreement with this convection and thus QPF totals through the day 1 period. Our axis of 1-3" stretches from central AR east into portions of central/northern MS/AL into northern GA (highest in MS/AL where some training is most likely). The latest WPCQPF noted a slight northward shift from the previous forecast. As far as the magnitude goes, again we leaned more towards the hi-res guidance, especially a blend of the WRF-ARW, ARW2, and HREF mean, which remain consistently higher than the global guidance, with peaks amounts in the 2-3" range (although locally could see upwards of 4"). Amounts much higher than this seem unlikely at this point given marginal PWATs/moisture transport and the progressive nature of the system. ...South and Central Florida... The presence of a mid level wave and the development of impressive upper level divergence will aide the widespread convection over the eastern Gulf of Mexico as it approaches central and southern Florida this evening. In general, we think the activity will be in a weakening state as it moves ashore given limited instability. It does, however, appear increasingly likely that at least some steady showers, and possibly a few embedded heavier convective cores, will move across southern Florida through ~ 06Z. WPC followed a multi model blend for QPF across this region...which is a decent increase from our previous forecast. For now kept areal averaged amounts below 1"...but as mentioned above will need to monitor trends as some chance we see locally higher totals. ...Southwest into the Southern Rockies... A wave moving into the Southwest will bring a broad area of precipitation into southern CA and inland across the remainder of the region and into the southern Rockies through the Day 1 period. An impressive plume of moisture is noted, with PWAT values at or near climatological max values for this time of year over the Southwest. This combined with the wave/left exit region upper jet streak forcing - certainly supports an expansive area of light to moderate precipitation amounts. Limiting factors will be the flatness of the wave / absence of strong deep layer lift, which is not really supportive of heavier rates, and also low level wind fields that are not all that anomalous. So while we certainly will see some orographic enhancement over southern CA, we are not expecting rates to be that impressive. Current indications are for rainfall rates to generally peak within the 0.25"-0.35" range within an hour across portions of the southern CA terrain this afternoon and evening. Days 2/3... ...West Coast... A strong mid level trough approaches the West Coast during days 2 and 3. Short wave energy ahead of the long wave trough taps Pacific moisture to produce locally heavy qpf across the higher terrain of CA and the Pacific Northwest during day 3. There was generally good model agreement with the timing of the synoptic scale systems, so the WPC QPF was based on a blend of the 12z ECMWF/GFS. Day 2... Ahead of the strong long wave trough slowly approaching the West Coast during day 2, a 40 to 50 knot low level southerly flow transports 0.75 inch precipitable water air more or less parallel to the CA coast. The moisture becomes focused on a warm front crosses northern CA, and local 0.10/0.25 inch qpf amounts were placed over portions of the CA coastal range. Day 3... As the long wave trough gets closer to the West Coast during day 3, it ejects short wave energy that crosses northern and central CA. The 45 to 55 knot low level flow veers to the southwest ahead of the short wave energy, transporting the 0.75 inch precipitable water air across the northern CA coastal range, as well as the northern Sierra Nevada mountains. The upslope flow is most efficient over the Sierra Nevada range, where the flow is nearly orthogonal to the terrain. There was a multi model signal for 2.00/2.50 inches of qpf along the northern and central portion of the range. Lowering snow levels should mitigate any flash flood threat on day 3 in this range. Lesser amounts (generally close to 1.50 inches) was placed over the northern CA coastal range. These amounts were also in good agreement with the model consensus. The upslope flow is not quite as efficient over the terrain of western OR/WA, but there should be enough lift with the short wave energy to produce a large area pf 0.25/0.50 inches of qpf here on day 3. ...Great Lakes... A long wave trough dropping south out of central Canada closes off in the mid levels over the upper Great Lakes during day 2. The closed mid level low then moves over western NY state and western PA during day 3. There was generally good model agreement with the movement and strength of the mid level low, so the WPC QPF was based on a blend of the 12z ECMWF/GFS. Some of the most recent NBM was used to better delineate lake effect precipitation each day. Day 2... As the long wave trough drops south from northern Ontario into the upper Great Lakes during day 2, it closes off in the mid levels. The main effect of the lowering heights is to steepen lapse rates over the upper Great Lakes as boundary layer conditions become more conducive for lake effect precipitation. The northerly low level flow drags colder air over Lakes Superior and Michigan, as mixed layer depths increase. The airmass in place is fairly dry, which could end up being a limiting factor for lake effect QPF. Local 0.10/0.25 inch qpf amounts were placed downwind of the lakes over the UP of MI, as well as the northern portions of LP of MI on day 2. Day 3... The low level north northwesterly flow increases early on day 3, as surface high pressure builds over the northern Plains. The flow favors downwind activity off Lake Superior, where local 0.25 inch qpf amounts were placed over the central UP of MI. The flow also favors lake effect activity off southern Lake Michigan, where a stripe of 0.10/0.25 inches of qpf was placed over far southwest MI and nearby northern IN. ...Ohio Valley/Mid Atlantic/Northeast... A positively tilted short wave tracks from the mid MS valley into the mid Atlantic states during day 2, developing into a long wave trough as its reaches the coast. Lift associated with the long wave trough spins up low pressure off the mid Atlantic coast late on day 2, which deepens as it heads for Nova Scotia during day 3. There is still a fair amount of model spread with the handling of the surface system, and its QPF impact each day. In an attempt to mitigate some of the differences, the WPC QPF was based on a blend of ensemble solutions (the most recent SREF, GEFS and ECWMF), then nudged toward the 12z GFS, especially during day 2. Day 2... A positively tilted short wave tracks from southeast MO across the lower OH valley into the western mid Atlantic states during day 2. Lift associated with the short wave, in conjunction with moisture on the low level flow ahead of it, is expected to produce an axis of 0.25/0.50 inches of qpf from southern IL across TN/KY into western VA/western NC. Additional short wave dropping into the short wave position carves out a long wave trough across the southern mid Atlantic late in the period, as it takes on a negative tilt. A low level southerly flow begins to transport 0.50/0.75 inch precipitable water air over southern VA/eastern NC and northern SC. Banding developing on the northwest side of the negatively tilted trough could produce a large area of 0.50/1.00 inches of qpf over southeast VA/central and eastern NC. Model soundings indicated that most of the elevated instability remains offshore. Finally, there is a fair amount of spread concerning how far north the qpf spreads during day 2. Both the NAM and GFS indicated shifts north with the northern edge of the QPF into MD/DE/southern NJ, and the 12z EMCWF shifted north as well. As mentioned previously, the WPC QPF was based mainly on a blend of ensemble solutions, in an attempt to even out the still volatile operational model landscape. A nudge toward the 12z GFS was employed over MD/DE/southern NJ, buoying confidence for higher amounts. Day 3... The surface low deepens off the NC coast early on day 2, but after that time there is still some model spread on how close the surface low and its attendant negatively tilted long wave trough track along the coast. The 12Z NAM/12z UKMET appear to be too far east, but the 12z ECMWF/GFS are closer with the track of both systems, and the ensemble model approach was modified by output from these models. The 0.25 inch qpf line was aligned from eastern NJ into western New England with the 0.50 inch line placed over eastern MA. Higher amounts are possible as banding on the northwest side of the surface and mid level systems could set up over eastern MA to produce amounts closer to 1.00+ inches here. If the model solution continues to converge for day 3, even higher amounts could be needed for eastern MA in later forecasts. Hurley/Hayes Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml