Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 433 AM EST Tue Feb 27 2018 Final Day 1, Day 2 and Day 3 QPF Discussion Valid Feb 27/1200 UTC thru Mar 02/1200 UTC Reference AWIPS Graphics under...Precip Accum - 24hr Day 1 Southern Plains---lower MS Valley---lower TN Valley The low level southerly to south southwesterly flow is expected to strengthen Tuesday across the Southern Plains and into the lower MS valley/lower TN valley by Tuesday night/early Wednesday. This will raise pw values to 2 to 2.5+ standard deviations above the mean and support increasing overrunning precipitation along and to the north of the northeastward moving warm frontal boundary across these areas. There is some model spread with axis of the heaviest precip---leading to below average confidence as the first wave of heavy precip breaks out---followed by a second potentially heavier event day 2. The day 1 qpf leaned toward the href mean which was closest to the more southern max axes of the hi res guidance and south of the EC...NAM and GFS axes. The heaviest precip potential likely late afternoon Tuesday into Tuesday night/early Wednesday corresponding to the strengthening of the low level inflow into the boundary. There will be potential for a period of training of cells to the north of the warm front with localized hourly precip totals in the .50-1"+ range. With stream flow anomalies much above average from recent heavy rains--additional runoff issues possible. The marginal and slight risk areas on the previous excessive rainfall potential outlook for this period were expanded slightly across northeast TX---southeast OK into central AR and southwest TN to cover some of the model spread with respect to the max axis. Lower MO Valley into the lower OH Valley Model consensus is for a secondary precip max farther to the north across portions of the lower MO river valley toward the lower OH valley ahead of a northern stream cold front pressing southeastward. With pw values also rising to much above average levels ahead of this boundary---moderate to isolated heavy precip amounts possible---although believe the most organized activity will be farther to the south along and to the north of the warm front where the strongest isentropic lift is expected. Southern California...southern Great Basin into the Southwest The well defined closed low over northern California early this morning will continue to press southeastward through central to southern California and into the Southwest this period. Strong westerly onshore flow into the peninsular range of southern California will support moderate to locally heavy totals. Elsewhere--the model consensus is for light to moderate totals from the southern Sierra into the southern Great Basin and Southwest as this system will continue to not have an anomalous pw tap with values near seasonal norms. Pacific Northwest into the Northern Rockies Weak onshore flow expected day 1 across the Pacific Northwest into the northern Rockies as shortwave energy passes southeast across southwest Canada. The day 1 qpf did not deviate from the model consensus of light to moderate totals from the northern Rockies of northern ID/northwest MT and through the Washington-northern Oregon Cascades west to the northern Oregon Coast range and the Olympic Range. Days 2/3... ...ArkLaTex/Mid MS Valley into East... ...MODERATE RISK for excessive rainfall on Day 2 (12Z Wed-12Z Thu) from northeastern TX through southern AR into southern TN, northern MS/AL and northwestern GA... A dynamic southern stream closed low, currently over central/southern CA, will eject out into the southern plains late Wed into Thurs. A frontal zone down over the northern Gulf of Mexico into south TX will return north over the next day or two and eventually stall. The current placement the guidance is indicating for this frontal boundary, appears to be from the ArkLaTex to the southern TN valley into the northern Gulf Coast states. This is where increasing Gulf moisture and perhaps even a subtropical feed will pool near this convergence zone for a possible heavy rain event. What makes this setup different from the past week, is this southern stream system is progressive and should advance downstream to eventually phase or interact with the digging northern stream. The trend in the two leading global models over the past day is a shift to the south with the zone for heaviest qpf. WPC followed this trend and mainly used a 00z gfs and 00z ecmwf blend, as 00z nam nest appeared to be an outlier or questionable. Thus a zone of areal avg 2 to 5 inch qpf amounts will be possible from nern TX through southern AR into northern MS/AL, southern TN and nwrn GA with locally higher amounts possible on Wed. As the upper dynamics and front move through the south and off the southeast coast on Thurs, a partial phasing or interaction with the southern stream and amplifying northern stream appears possible. This may allow some Atlantic inflow to spread across the northern Mid-Atl region back to a dynamic northern stream vort/possible closed 500mb low. Thus an axis of 1-2 inch areal avg qpf amounts will be possible and WPC introduced a marginal threat for excessive from parts of NJ/DE through PA, as FFGs are extremely low due to saturated conditions. Also, a corridor from southern WI/northern IL through Lower MI into western NY/nwrn PA may see some decent snowfall accumulations and please refer to the latest qpfhsd for more on the frozen precip. ...West... Another phased cyclone will drop down the West Coast on Wed and Thurs to produce heavy mountain snow and possible heavy rainfall along coastal sections of WA to CA. A short wave crossing the Aleutians and another digging through AK will begin to phase over the nern Pacific and near the British Columbia coast on Wed. A large closed 500mb circulation will form and a steady increase in onshore flow will begin to consume the Pac NW. In fact, a slight anomalous plume of Pacific moisture transport appears to reach the WA/OR coast into northern CA late Wed into Thurs morning. This may help yield 1 to 2 inch areal avg amounts for the coast and coastal ranges on Wed. The entire closed low will continue to dig south on Thurs, the associated frontal zone will sweep inland and the slight anomalous plume of Pacific moisture will drop down almost the entire CA coastline. Thus a marginal threat for excessive rain will be possible from central to southern coastal/coastal ranges and in some burn scar areas for runoff issues. Otherwise, most of the moisture will flourish in the Sierra with ideal for orographics and heavy mountain snow. WPC stayed very close to the reliable gfs and ecmwf for qpf amounts. Oravec/Musher Graphics available on the web at www.wpc.ncep.noaa.gov/qpf/qpf2.shtm