Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 402 AM EDT Wed May 23 2018 Prelim Day 1, Day 2 and Day 3 QPF Discussion Valid May 23/1200 UTC thru May 26/1200 UTC Reference AWIPS Graphics under...Precip Accum - 24hr Day 1 ...Southeast and Mid Atlantic... Scattered convection is once again expected along and south of a southward moving cold front. A few different forcing mechanism are expected to locally focus convection and produce some higher areal averaged amounts. The first area of focus will be across eastern North Carolina where low level convergence along the front should be strongest. Enough shear here for some organization of convection near the front. Storm motions should generally be off to the south, but not all that quick, and some regeneration near the front may lead to some brief training. The 0z HREF depicts this scenario well, with moderate probabilities of localized 3" amounts focused across the area. Another area of focus will be the lingering low/mid level low over AL/GA. As expected, thunderstorms have concentrated near the center of this system early this morning, producing a narrow swath of heavy rains near the AL/GA border. With diurnal heating expect convective activity to expand outward from the center as has happened the past few days. Again these cells will generally be pulse in nature and short lived. Although continued high PWATs support brief heavy rates, and wherever cell mergers along outflows occur, a localized flash flood risk may exist. Will see a contraction of activity towards the center after sunset over south GA. Although in general the model consensus seems to suggest the focus Wednesday night will not be as concentrated/heavy as what is occurring early this morning. May also see the terrain of the western Carolinas act as an initiation focus for storms this afternoon. Although with drier air moving in from the northwest the signal is not as strong as previous days. Thus cells may generally be more isolated in nature and short lived. Nonetheless some very localized heavy rates will be possible. ...Lower MS Valley... The Lower MS Valley will be a focus for convection again today. A weak shortwave trapped underneath the mid level ridge axis will act as forcing mechanism for storms. Plenty of diurnally driven instability forecast, and PWATs will remain well above normal. Storms will be of the pulse variety and will generally be short lived, however weak mean flow suggests slow cell motion. Will probably generally see chaotic cell motion with storms forming along outflows and quickly dissipating. The 0z HREF signal is a little more robust than yesterday, so anticipate convection will be a bit more widespread today, resulting in higher overall areal averaged QPF. Not expecting any organized flash flood threat, although where some brief cell mergers occur extending rainfall duration, anticipate we could see a few localized flash flood concerns this afternoon over AR. ...Northern Rockies into the Plains... Elevated convection likely to continue into the morning hours across portions of southern MN and northern IA, with locally heavy rains possible along a northwest to southeast training axis. This activity should generally weaken as we go through the morning hours with the diurnal loss of the low level jet into the front. Thereafter details with the convective evolution across this region is low confidence. Will likely see some storms form in the unstable airmass near the northward lifting warm front across SD/ND...although at this point not much of a signal for organized activity. Also will see storms fire along the dryline over the western Plains by afternoon. This activity will move eastward into the overnight hours. Given the instability over the Plains into the overnight, seems reasonable that some of this dryline convection may survive eastward as a cluster or two. Suspect the 0z high res guidance are probably too weak/isolated with this evening/overnight activity, although confidence in the details is low. One area where there is a bit more of a high res signal for organized convection is across eastern NM into west TX. This area does have a few factors going for it...one being some enhanced upper level divergence in the left exit region of the subtropical jet, and the other a bit of an increase in southeasterly 850 mb moisture transport. So will need to watch this area this evening for some potential locally heavier rainfall amounts. A good signal for some 1" areal averaged rainfall across portions of northwest WY into central MT. This area will see the strongest dynamics and above normal moisture. Instability will be limited here, thus generally think we are looking at more of a steady stratiform rainfall, maybe with a few embedded heavier convective cores. ...Northwest... Widespread shower and thunderstorm activity expected across portions of the northwest in the broad troughing. The focus for heaviest amounts appears to be across the terrain of northern CA into the OR Cascades, and portions of central and eastern OR. Cells will generally be of the pulse variety and short lived, limiting overall areal averaged rainfall. PWATs will be around climatological maximum values though, and thus would anticipate some localized heavy rates. The best chance for some heavier totals is probably along/near the Cascades, where the terrain will likely act as forcing mechanism and result in some repeat cell activity. Days 2/3... ...Southeast/eastern to central Gulf Coast/FL... Deep moisture within confluent flow encounters a cold front moving across the southeast on Thursday. Diurnal heating produces an areal average QPF max of 1-1.25 inches near the southern semicircle of the remnant 850 mb circulation in GA. Elsewhere expect locally heavy showers in areas of diurnal instability and sea/lake breeze initiated lift and convergence near the front. 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, and this is the case as both QPF was based primarily on the 00z GFS and 00Z ECMWF which came into better alignment over the central Gulf coast. The models are starting to converge a little closer on the evolution of the offshore gulf cyclonic circulation. The 00Z NAM slowed down its forward motion to be closer to the ECMWF but is stronger. Given the other models have slower forward motion(further offshore), the idea was to have slower timing on the arrival of heavier showers than the 00z NAM. ...Pacific Northwest/Great Basin/Northern Rockies... A closed mid-upper level low off the CA coast through Thu night drifts slowly east towards CA. 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. On Fri 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. Manual progs blended the NAM/GFS/UKMET and 12-00z ECMWF QPF. ...Central Plains to the mid-upper MS Valley and upper Great Lakes... Activity will focus ahead of a low level front and pre-frontal moisture plume extending from northern KS across eastern Nebraska, southeast SD, northwest IA and MN on day 2. Clusters of showers and storms are expected again with convergence on the nose of the low level jet progressing these areas. Manual progs blended the 00z UKMET/GFS/ECMWF QPF. On day 3, Fri, 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 from day 2 brings the likely targeted areas for shower/thunderstorms development from southeast KS across MO, near the IA/IL border, WI, and the UP of Michigan. QPF was derived by blending the ECMWF, GFS, UKMET, and NAM QPF. Chenard/Petersen Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml