Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 456 PM EDT Wed May 23 2018 Prelim Day 1, Day 2 and Day 3 QPF Discussion Valid May 24/0000 UTC thru May 27/0000 UTC Reference AWIPS Graphics under...Precip Accum - 24hr Day 1 ...Lower MS Valley---central Gulf coast into the Southeast/southern Mid-Atlantic,,, Widespread scattered diurnally enhanced convection likely day 1 along and to the south of the nearly stationary frontal boundary expected to lie from the Lower MS Valley---across the central Gulf coastal region and into the Southeast/southern Mid-Atlantic. With pw values expected to remain much above average along and south of this front---locally heavy precip totals possible---although confidence is low with details. One region of potential max precip will be in the vicinity of the nearly stationary surface to mid level circulation in the vicinity of central Georgia. Precip may focus near this center and then to the west southwest and northeast along the above mentioned frontal boundary position and/or surface trofs separate from the frontal boundary positions emanating from this nearly stationary low. ...Northern Rockies--- Northern Plains into the Upper MS Valley... Height falls pushing northward Wednesday across the Northern Rockies will move on a more east northeast track Thursday across the Northern Plains and toward the Upper MS Valley. The model consensus is for the well defined comma head/deformation precip area currently stretching from the Northern Rockies into the Northern High Plains to weaken at the beginning of the day 1 time period with more scattered precip near the primary height fall center over the Northern High Plains. At the same time---broadly difluent upper flow downstream of these height falls will support scattered convection from the Northern Plains into the Upper MS Valley. Areal average moderate precip totals depicted with locally heavier amounts likely where convection maximizes. ...Central to Southern High Plains... A lee trof/dry line will extend across a large area from the Central to Southern High Plains day 1. The broadly difluent upper flow expected across the Northern Plains/Upper MS Valley will also extend southward across the Central to Southern High Plains. This will support widespread scattered convection---primarily during the first 6 to 9 hours of the day 1 period tonight and then again late Thursday afternoon. Confidence in qpf details is low in this flow regime with a myriad of model solutions. The HREF mean was used to mitigate the model to model and model run to model run differences. Moderate areal average amounts depicted with locally heavier totals likely where convection maximizes. ...Northern California---portions of the Pacific Northwest---Northern Great Basin into the Northern Rockies... Widespread diurnally driven shower and thunderstorm activity likely early in the upcoming day 1 time period and again late in the period during Thursday afternoon. PW values expected to remain above average across these regions in the wake of the lead area of height fall ejecting north into the Northern Rockies early day 1 and ahead of the developing closed low off the California coast late day 1. Generally light to moderate totals expected across these areas--with isolated heavier totals possible. Days 2/3... ...Southeast/eastern to central Gulf Coast/FL... Deep tropical moisture within confluent flow encounters a weakening cold front over the southeast into Thursday night. Diurnal heating produces an areal average QPF max of 1.5 inches in eastern GA/southern SC east of a remnant 850 mb circulation. PW anomaly of 2.5 immediately east of the 850mb circulation warrants a Slight risk for excessive rainfall for Day 2 (12Z Thu-12Z Fri) with a surrounding Marginal risk. Expect locally heavy showers in areas of diurnal instability and sea/lake breeze initiated lift and convergence near the front, particularly back over southeastern AL to the FL panhandle. The models also depicted persistent, deep flow of moisture from the Bahamas into southeast FL with peak precipitable water values near 2 inches so showery conditions are expected along the southeast coast. The pattern favors inland penetration of the sea breeze from the east coast so peak afternoon convection should occur over interior portions of the FL peninsula to the west coast. Lapse rates are not as high as usual due to warm mid level temps, so bands of larger scale lift are needed to trigger FL convection. Convective activity into southern FL warrants a Marginal risk for excessive rainfall for Day 3 (12Z Fri-12Z Sat). QPF was based primarily on the 12Z GFS/ECMWF which are in good agreement. The 12Z ECMWF continues a trend of slower arrival of tropical moisture and circulation. The 12Z GFS continues a much farther east track as if follows convective trends to FL. Much uncertainty remains with this activity, but as noted, there is an expanse of tropical moisture across the southeast, leading to at least locally heavy rainfall. ...Pacific Northwest/Great Basin/Northern Rockies... A closed mid-upper level low off the CA coast Thursday night drifts to NV through Saturday. Southerly flow ahead of the circulations increases moisture with 700 mb convergence driving rain and shower across northeast CA and northwest NV across into OR. PW increases to around 1 inch ahead of the low over parts of OR which is 2 standard deviations above normal, so expected amounts are above normal for May. Thursday activity is expected to propagate up the CA Cascades to OR. Slow storm motion warrants a Day 2 (12Z Thu-12Z Fri) Marginal risk for excessive rainfall along the CA/NV border into OR. On Friday the deep layer circulation drifts onshore in CA with the upper trough drifting east across OR. Well defined coupled 700 mb convergence and 300 mb divergence cross from northern Ca and NV into OR, with better than average cluster of higher QPF across southern OR as a result. With the east drift of the upper trough, the peak upper divergence drifts into the OR Blue Mountains, so a secondary maxima is expected there Fri/Fri evening. A Marginal risk for excessive rainfall is in effect for much of southern OR. Good agreement among global guidance nearly warranted a Slight risk, though an overall lack of instability may limit rainfall. QPF was based on The 12Z ECMWF/GFS. ...Central Plains to the mid-upper MS Valley and upper Great Lakes... Activity will focus ahead of a low level front and prefrontal moisture plume extending from northern KS across eastern Nebraska, southeast SD, northwest IA and MN Thursday into Thursday night. Clusters of showers and storms are expected again with convergence on the nose of the low level jet progressing these areas. Given the anomalous moisture plume pushing across the warm front (1.5 inch PW is 2.5 standard deviations above normal), a Marginal risk for excessive rainfall was raised for Day 2 (12Z Thu-12Z Fri) over central MN. This area will need to be monitored. It should be noted that hi-res CAMs did not handle convective activity well Wednesday morning across this area. On Friday, the 700 mb trough move east from the northern and central Plains towards the mid-upper MS Valley. Lift from the approaching wave within the low level moisture plume and modest low level jet maxima supports more showers/storms where instability is focused. The east drift of the low level jet and moisture plume and upper trough difluent flow brings the likely targeted areas for shower/thunderstorms development in a corridor from southeast KS to the UP of Michigan. A cold front over Ontario is the terminus for tropical moisture, so much of the heavy rain in Day 3 will occur in Canada. QPF was derived by blending the 12Z GFS/ECMWF. Oravec/Jackson Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml