Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 342 AM EST Wed Feb 28 2018 Prelim Day 1, Day 2 and Day 3 QPF Discussion Valid Feb 28/1200 UTC thru Mar 03/1200 UTC Reference AWIPS Graphics under...Precip Accum - 24hr Day 1 Southern Plains---lower AR river valley into the TN Valley-southern Appalachians There continues to be a strong model signal for widespread heavy to excessive rainfall amounts from far northeast TX/far southeast OK---into southern to central AR---northern MS---much of TN---northern AL and into the southern Appalachians. The initial round of heavy precipitation that has moved across these regions Tuesday into Tuesday night/early Wednesday will be followed by another surge of organized overrunning convection to the north of the northeast moving warm front from the lower MS valley into the TN valley and frontal convection ahead of the advancing cold front through the Southern Plains into the lower MS valley. Low level inflow of much above average pw air will be re-focusing into the warm front Wednesday afternoon into Wednesday night ahead of strong mid to upper level height falls pushing into the Southern Plains. This will be followed by developing frontal convection Wednesday night which should push into the lower MS valley by early Thursday. With the above mentioned strong moist inflow into the boundary and very favorable right entrance region jet dynamics---confidence is high for a widespread heavy to excessive precip event. There is only a small amount of model spread with respect to the qpf axes. The nam is the more northern outlier with much of the remainder of the guidance and the WPC forecast continuing to favor a farther south solution. Changes to the previous WPC qpf were relatively small---a slight increase in precip totals on the north side of the max axis. Excessive rainfall will remain a significant threat this period. Rains over the past week have raised stream flow to much above normal to high level as per the national water model with the additional expected totals likely to produce additional flooding concerns. No significant changes made to the previous excessive rainfall potential outlook across these area for this period. A very large slight and moderate risk area remains from northeast TX---far southeast OK--far NW LA---central to southern AR---northern MS---northern AL---much of TN into far northern GA---far southwest NC and far northwest SC. Mid MS Valley---OH Valley To the north of the expected convective max from the southern Plains---lower AR valley into the TN valley---a broad region of moderate precip amounts likely from the mid MS valley into the OH valley and just south of the lower lakes in a region of strengthening isentropic lift ahead of surface low pressure moving east from the mid MS valley to north of the OH valley. Precipitation amounts to the north of a well defined convective max can often difficult for models to handle---with a lot more spread evident in the latest runs leading to lower confidence in qpf details here. WPC qpf leaned more toward the href mean and in house pseudo bias corrected mean to mitigate some of the model detail spread with areal average .25-50" amounts depicted. Northern NY state into northern New England Northern stream shortwave energy and the associated surface wave pushing from the upper lakes into New England will support a narrow band of moderate precip amounts from northern NY into northern New England. Accumulating snow likely confined to northern Maine. See the latest qpfhsd for additional winter weather information. Southwest into the southern Rockies The strong area of height falls that enhance heavy rain potential over the southern Plains---lower AR and TN valleys today will support scattered precip Wednesday over the Southwest into the southern Rockies. Model consensus is for light to locally moderate areal average precip totals with WPC qpf not deviating from this. Pacific Northwest into northern California Strong amplification expected to the mid to upper level trof dropping southeast from the Gulf of AK/northeast Pacific region to a position off the B.C./Pacific Northwest coast by the end of the day 1 period. This developing closed low will have a better tap of above average pw values than the previous systems that have amplified into the mean western trof over the past several days. Strengthening southwest onshore flow will support widespread heavy precip totals this period from the northern Sierra/northwest California coast range---northward along the Oregon coast range and into the Olympic range. Heavy precip values likely inland through the Oregon Cascades---while moderate to heavy totals expected north into the Washington Cascades. The heaviest precip totals likely from the northwest California coast range into the southwest Oregon coast range where the onshore flow will have the best upslope component. A marginal risk of runoff issues was maintained here from previous forecasts. Oravec Days 2/3... ...East to Northeast... Following the heavy to excessive rain event over the southern tier of the country on Wed, the remaining southern stream upper dynamics and front will move through the south and off the southeast coast on Thurs. Meanwhile along the northern stream, a rather active short wave with a slight neg tilt will move into the Great Lakes and OH Valley, with a surface low transitioning from the western OH Valley to Lake Erie and nwrn PA/western NY. A partial to full phasing or interaction with the two streams appears likely but enough spread exists to make the forecast a bit uncertain. The phasing would allow a low near the nern seaboard to deepen rapidly and go through a full cycle of cyclogenesis, while spreading a wealth of Atlantic inflow to the west. Perhaps a rare Nor'easter for this winter with the combination of heavy rain and a marginal threat for excessive amounts along with wet snow. The two leading global models are inching a bit closer tonight on this scenario but not entirely in full agreement. WPC followed a blend of the two with the 00z nam nest and internal pseudo bias corrected guidance for Thurs and a gfs/ecmwf blend/compromise on Fri. The qpf amounts with the fast moving southern stream feature will likely yield 1 to 2 inch areal avg amounts within a tight gradient from northern AL/MS through the southern Appalachians and much of the state of NC on Thurs. Meanwhile, moisture initially with the northern stream will be a bit limited over the first half of the period on Thurs but serly Atlantic inflow will up moisture content the second half. Thus expect dynamic cooling rain/snow across the Great Lakes and northern OH Valley, with rain mainly over the northern Mid-Atl states. Then on Fri, with the low pressure deepening tremendously just offshore, a compromise was used with moisture spreading along an inverted surface trough through NY/NJ to Lake Ontario and intense onshore flow into coastal New England. ...West... ...SLIGHT RISK for excessive rainfall on Thurs (12Z Thu-12Z Fri) for the CA coast from the Southern Coast Ranges to the Transverse Ranges... Another phased cyclone will drop down part the West Coast on Thurs and Fri to produce widespread heavy mountain snow and possible heavy to excessive rainfall along coastal sections of CA. Multiple short waves over the nern Pacific and near the British Columbia coast will phase on Wed. A large closed 500mb circulation will form and dig south on Thurs, the associated frontal zone will sweep inland and a slight anomalous plume of Pacific moisture will drop down almost the entire CA coastline. Thus a slight threat for excessive rain will be possible and kept in the forecast from central to southern coast/coastal ranges and in some burn scar areas. Thus runoff issues will be possible with associated mudslide threat. Otherwise, most of the moisture will flourish in the Sierra with ideal for orographics and extremely heavy mountain snow. Then on Fri, the closed low will fill while moving inland over OR and the baroclinic zone will press east into the Inter-Mountain West into southern CA. Overall WPC stayed very close to the reliable gfs and ecmwf for qpf amounts each day. Oravec/Musher Graphics available on the web at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml