Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 608 AM EDT Wed Apr 04 2018 Final Day 1, Day 2 and Day 3 QPF Discussion Valid Apr 04/1200 UTC thru Apr 07/1200 UTC Reference AWIPS Graphics under...Precip Accum - 24hr Day 1 ...Great Lakes---Northeast---Eastern Seaboard... The strong closed low moving into the lower Great Lakes Wednesday morning is expected to progress steadily northeastward across southeast Ontario into southern QB Wednesday and northeast into eastern QB Wednesday night/early Thursday. The well defined comma head/deformation precipitation band to the north of this closed low may linger early day 1 over the northeast portion of the L.P. of MI---but remain mostly north of the border across portions of southeast Ontario and southern Quebec. Strong isentropic lift ahead of this closed low will also support heavy snow potential over far northern Maine. In the wake of this system moving northeastward---west northwest cyclonic flow will persist across all of the Great Lakes day 1. This will support scattered snow showers down wind of the lakes and into upslope of the central Appalachians. The trailing surface cold front emanating from the above mentioned low will push quickly eastward Wednesday through the northeast/New England and off the Eastern Seaboard. The fast movement of the front will be a limiting factor for significant precip amounts with model consensus for .10-.25" amounts from eastern NY state---central to southern New England---south through the Mid Atlantic---Southeast and northeast Gulf Coast. ...Pacific Northwest into the northern Rockies... PW values will be increasing to 1 to 2 standard deviations above the mean day 1 over the Pacific Northwest into the northern Rockies as west southwest to southwest onshore flow persists across these regions. This will support heavy precipitation potential from the Washington Cascades west to the Washington coast range and Olympic Range. Moderate to isolated heavy precip totals possible inland through the northern Rockies from northwest WY into western MT---and northern ID. Days 2/3 ...Pacific Northwest/California... After a fairly short respite from the recent wet pattern over the Pacific Northwest, another round of heavy to potentially excessive precipitation looks to get started by day 3...especially over portions of northern California but extending northward into portios of Oregon. Once it begins, the event will continue into the Medium Range section. Deep low pressure forms over the waters of the eastern North Pacific, which helps to direct a plume of deep layered moisture/atmospheric river into the west coast of North America with the axis aimed over northern California. Some generally light precipitation will begin as early mid- to late-day on Thursday once the nose of the moisture plume approaches and H85 flow accelerates to around 25 kts. Precipitation rates increase substantially on Friday afternoon as the precipitable water values increase to around 1.75 inches which is being directed onshore by H85 winds between 35 kts and 45 kts. PW values of 1.5 inches would be in excess of 6 standardized anomalies above climatology while the combination of moisture and wind result in 850 mb moisture transport anomalies more than 5 standardized anomalies above climatology. By the end of day 3, the highest values will start to be shunted southward into portions of central California as the North Pacific low heads north and its associated cold front moves inland. Given the amount of precipitation forecast falling in the terrain, we hoisted a slight risk of excessive rainfall that extends from the coast eastward as far as the upslope region of the Sierra Nevada ranges. Considering that snow levels should be fairly high and increasing in response to ridging ahead of the system, the slight risk was drawn fairly high up in the upslope region of the Sierras. Based on guidance from the WPC model diagnostician, we tended to put more weight on the timing and track of the synoptic scale features from the ECMWF. One caveat here, though, is that the ECMWF QPF seemed to be too light considering the anomalous amount of moisture/moisture transport so we tended to increase amounts closer to the GFS. Some ensemble members also supported the heavier amounts. An upgrade may be needed later. ...Northern Rockies into the Middle/Lower Mississippi Valley... A cold front sagging southward from Canada will become quasi-stationary as the cold air dams up against the Rockies...while the southern portion of the front makes its way into the the Southern Plains/Lower Mississippi Valley. Moisture spilling over the top of the continental divide will get an additional boost as it rides up and over the cold domem. The result will be a swath of precipitation that extends out over the plains. Some amounts here could approach half an inch. Additionally, moisture from the Gulf of Mexico will stream northward over the Southern Plains into parts of the Lower Mississippi Valley with precipitable water values of 1.25 to 1.50 inches from Friday into Saturday. WPC used a blend of the GFS and the ECMWF as a starting point for the manual guidance although more weight was given to the GFS by the end of the period. ...Southern Plains to Tennessee Valley and the Mid Atlantic... Surface low pressure begins to develop over portions of the Southern Plains to the Tennessee Valley on Friday, fueled by the push of arctic airmass from the north and a warmer...albeit modified...airmass to the south. Precipitation will be breaking out along north of the front as the moisture ascends isentropically over the cold dome and aided by the surface isallobaric falls. By Friday evening and Friday night, a broad expanse of precipitation should be developing from the Southern Plains towards the Mid Atlantic aided by the right entrance region of a 80 to 110 kt upper level jet that builds back into the Tennessee Valley. The 00Z GFS was most bullish with the precipitation coverage, but the idea was similar in the various global models. A general model compromise...shaded somewhat towards the GFS...was favored in this part of the country. Oravec/Bann Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml