Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 548 AM EDT Thu Apr 05 2018 Final Day 1, Day 2 and Day 3 QPF Discussion Valid Apr 05/1200 UTC thru Apr 08/1200 UTC Reference AWIPS Graphics under...Precip Accum - 24hr Day 1 ...0830Z Issuance... Only some minor modifications were made in the Northern Rockies and around the Eastern Great Lakes to better fit the short-term radar. In each case, the 06Z issuance had allowed for precipitation to taper off but the radar imagery suggested that it may have been premature. Overall, though, the fundamental forecast reasoning did not change significantly from the HPCQPF issued at 06Z. ...Pacific Northwest into the northern Rockies... An impressive surge of moisture pushes into coastal Oregon/Washington later this morning ahead of a cold front. Precipitable water values of 0.75-1.00" (+2 to +2.5 standard deviations) will support widespread light to moderate amounts, generally 0.25 inches to 0.50 inches, but a bit lighter in the valleys and about 2-3 times that in the Olympics/Cascades. The nose of a 110kt jet streak should push into southwestern Oregon by early afternoon coinciding with larger PW anomalies (southward to NorCal) of about +2 to +3 standard deviations. The ARW2.5 and ARW5 cores tended to fit pretty well with the 00Z global model runs over much of the region. As a result, we tended to give more weight to a consensus of the ARW and employed them in order to provide better horizontal resolution. A quick look at the HREF mean (or blended means) showed it was still a viable solution. The end result was a QPF that was not too different from continuity across the Pacific Northwest into the Northern Rockies. Much like on the day shift, followed the idea of expanded QPF area over northern ID into MT/WY given the rising PW values, a ~120kt jet out of SW Canada, and the surface front along/east of the Divide with modest upslope potential. ...Florida... Some lingering showers are possible along the southeast coast of the Florida peninsula this afternoon and evening near a weakening cold front. Hi-res models still target extreme southern portion of the peninsula given northeast winds behind the front (sinking into the Florida Straits) and sea breeze development. Isolated local maxima may approach 0.5 inches but the heaviest rainfall and best coverage of precipitation should be off shore. ...Southern Plains... Southerly flow should be developing over the southern plain during the afternoon and evening. These winds will tap some moisture and start to draw it northward which leads to showery precipitation breaking out over parts of the Southern Plains. Initially, the model agreement is not particularly good, but once the H85 flow accelerates to around 30 kts and the nose of the 1.00 inch precipitable water value reaches northeast Texas/northwest Louisiana between 06/06Z and 06/12Z. Days 2/3 West Coast: An atmospheric river is forecast to bring abundant precipitation to much of central and northern California from Friday morning through Saturday afternoon. Anomalous PWs on the order of 1.5 to 1.8 inches will be advected into the region by strong WSW low level flow with a tropical connection, aided by a 150kt upper level jet. With lower tropospheric flow oriented orthogonal to the mountainous terrain, orographic lift will be enhanced and the result will be several inches of rain for some locations between San Francisco and the Oregon border, and also for the lower elevations of the Sierra. The slight risk area has been maintained for this region for Day 2, with the greatest flooding potential over burn scar areas. The precipitation is forecast to abate significantly by Saturday night as the atmospheric river weakens and moves away from the coast. WPC used a blend of the GFS, ECMWF, and biased corrected model for the QPF forecast with adjustments made for the terrain. Deep South to the East Coast: A strong frontal boundary is expected to become established across the southern plains and extending eastward to Tennessee and Virginia as a late season arctic high settles southward across the central and northern plains. Surface cyclogenesis is expected along this front over Texas Friday afternoon and then exit the southeast U.S. coast by Saturday night. An enhanced warm conveyor belt ahead of the low will advect deep moisture northward and intercept the quasi-stationary front, with PW values near 1.5 inches in a zone of strong moisture convergence. A mid-level shortwave embedded in the large-scale synoptic trough will further enhance lift, and QPF on the order of 1 to 3 inches is likely from the ArkLaTex region to northern Mississippi on Day 2, and a slight risk area for excessive rainfall was introduced for this area. The winter weather discussion (QPFHSD) has additional information regarding the snow potential from this event across the Appalachians and Mid-Atlantic region. Bann/Hamrick Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml