Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 610 PM EDT Sat Apr 07 2018 Final Day 1, Day 2 and Day 3 QPF Discussion Valid Apr 08/0000 UTC thru Apr 11/0000 UTC Reference AWIPS Graphics under...Precip Accum - 24hr Day 1 ...Pacific Northwest/Northwest CA... A strong closed mid level low off the Pacific Northwest coast moves onshore and weakens as it moves onshore and inland near the WA/BC border this evening. Low level moisture fluxes on the nose of an 850 mb jet forecast to be 50-60 kt by the 12z gfs and upper level divergence combine with upslope flow to produce heavy to locally excessive rainfall over windward terrain tonight. There was good model agreement with the evolution of the mid level system, so the WPC QPF was based primarily on a multi-model and ensemble blend, including the 12z HREF mean. There was a multi model signal for an axis of 2.00 inches of qpf in the OR Coastal range into the Olympics in WA, and also in the southern WA and northern OR Cascades. This much rain could result in local runoff issues, especially across the coastal ranges late in the period. The strength of the low level jet and resultant low level convergence drops as you head south across northern CA, so rainfall amounts subside in concert. ...Rockies/Great Basin/Northern Plains... Short wave energy in a fast mid level flow provides synoptic scale ascent over a frontal boundary extending from western MT to WY, and surface low pressure develops on the front over WY. The combination of moisture from a Pacific plume, upslope flow and synoptic scale lift is expected to produce locally heavy qpf amounts over the CO Rockies. There was good model agreement with the overall synoptic setup, so the WPC QPF was based a multi-model and high res ensemble (HREF) mean over the terrain. The initial activity has developed over UT, which persists into the evening until it moves east overnight in tandem with the upper jet existing UT and crossing CO. Precipitable water values over the the CO Rockies rise to between 0.50-0.75 inches, which is 3-3.5 standard deviations above the mean. A sfc low pressure that develops over WY is forecast to move east southeast to near the SD/NE border Sunday. Precip develops in the band of low level frontogenesis north of the low track....with the models indicating a quarter to half inch liquid equivalent across northeast MT to the SD/ND border and then southwest MN Sun. See the latest winter weather graphics and discussion for potential snow amounts. ...Southern Mid Atlantic/Southeast coast/Florida... Short wave energy spins up low pressure on a frontal boundary over SC. Ahead of the front and surface low, moisture and instability will feed convection that produces locally heavy rainfall on the southeast coast. A trailing cold front moves east across GA and into the coastal waters tonight and south across the Fl peninsula tonight and Sunday. Ahead of the front, a 40 knot low level southwest flow focuses 1.50 inch precipitable water over FL/GA into eastern SC and NC. Divergence in the right rear quadrant of a 150 knot jet streak will help organize the convection, which moves off the coast this evening. The front drifts south across central Florida Sunday, with showers and storms developing, supported by low level moisture convergence along and south of the front, aided by sea and lake breeze boundary convergence and deep moisture with forecast soundings indicating layer sfc-700 mb relative humidity of 70-80 percent. With deep westerly flow, afternoon convection moves east towards the east coast. Slightly more weighting was given to the 12z NAM maxima along the southeast coast. Days 2/3 ...Pacific Northwest to the Plains... Mainly light precipitation will linger Sunday night from the Northwest into the Rockies. Mid/upper level ridging will build in Monday ending precipitation here. Generally followed a multi model blend for QPF. Did note some QPF differences across the Central Plains as weak shortwave energy ejects eastward on Monday. Leaned this forecast a bit more towards the ECMWF/UKMET which showed an axis of slightly higher totals from eastern CO into Kansas ahead of the wave. A cold front and mid level shortwave energy will push into the Pacific Northwest late Monday night into Tuesday...bringing a return to the precipitation threat. Some timing differences noted here (although the 12z runs seem to be trending towards a consensus for a quicker solution)...and for now preferred to stay close to a general model blend...with the GFS/ECMWF/NAM/ENS BC used to derive QPF. ...MS Valley into the Mid Atlantic... Light snow will be ongoing Sunday evening over portions of the mid/upper MS Valley...with this system weakening as it moves towards the Mid Atlantic on Monday. Some model differences with the exact magnitude of amounts with this system...but either way only light totals expected given the weakening forcing and limited moisture. WPC QPF blended the GFS/ECMWF with some incorporation of the high res guidance early in the day 2 period. ...Southeast... On day 2 (0z Mon-0z Tue) likely to see showers and embedded thunderstorms over central and northern Fl in the vicinity of a stalled out front. Light rainfall is possible north into eastern GA and portions of the Carolinas. The 12z NAM remains an outlier developing a stronger low off the southeast coast. This solution has little to no support form the other models and thus is not preferred. Thus WPC followed close to the GFS/ECMWF/UKMET. On Monday night into Tuesday the main trough axis pushes into the Southeast. Better mid/upper level forcing will accompany this trough...with some right entrance region upper jet dynamics in play as well. Thus would expect the front over FL to lift a bit north, with increasing PWATs likely as well. Thus Tuesday should see a greater coverage of showers and locally heavy thunderstorms moving across central/northern FL. Related to issues noted above with the NAM, continued to lean away from its solution on day 3. Even amongst the GFS/ECMWF/UKMET noted some north/south rainfall axis differences (although the southward trend in the 12z ECMWF brings it into closer agreement with the UKMET)...but the magnitudes amongst these three models were similar. Opted to blend them together for now, resulting in a 1-2" swath of rainfall...with locally heavier totals probable. ...South TX... A southward shifting frontal boundary will likely become convectively active on Tuesday across northeast Mexico towards the border with south Texas. Current indications are that the heavier totals will remain south of the border, but will continue to monitor trends. Petersen/Chenard Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml