Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 556 PM EDT Sun May 13 2018 Final Day 1, Day 2 and Day 3 QPF Discussion Valid May 14/0000 UTC thru May 17/0000 UTC Reference AWIPS Graphics under...Precip Accum - 24hr Day 1... ...Midwest to Mid-Atlantic... Instability was developing near a somewhat wavy frontal zone across the Ohio Valley and Mid Atlantic. The synoptic background remained loosely favorable for diurnal convective development, with flat cyclonic flow and slight 12 hour 500-mb height falls, despite this being quickly followed by warm advection and height rises later tonight. Thunderstorms initiating in the high terrain of eastern OH into WV/PA and northern VA may become locally organized and evolve eastward, though persistent cloud cover near the coast will likely yield a weakening trend to anything that tries to move toward eastern Maryland up through New Jersey. Meanwhile, farther south in Virginia there may be insufficient forcing to produce much more than isolated storms. Back west a remnant convective cluster may find new life as it exits eastern Iowa and eventually taps into the growing surface based instability over the region. This cluster/system could hold together through evening before weakening. The pattern is then primed for another nocturnal event to develop over the Midwest - Iowa into IL/WI. The 12Z model suite came into very good agreement as to this, with many of the individual models developing a progressive MCS / bow echo by late tonight, and one that could survive through the day tomorrow - a very summer-like event if it occurs. There is a lot of model evidence for this, and the consensus QPF increased somewhat along the path of this expected MCS. The current pattern has consistently verified on the north side of guidance given warm low to mid level temperatures and marginal low level moisture, so we leaned north a bit, especially from Lake Michigan eastward where we favored the WRF-NMMB and NAM CONUS Nest. The QPF crosses some sensitive areas with wet antecedent conditions and high river stages, and we did introduce a Slight Risk of excessive rainfall for parts of IA/IL/WI. ...Florida... Again a difficult forecast here. An injection of tropical moisture around the periphery of a broadly developing baroclinic trough yields the potential for heavy rain. The models are depicting very heavy downpours anywhere that lift occurs, given the tropical thermodynamic profiles. Today, however, any substantial instability is located offshore, and rainfall is rooted in the synoptic lift within the middle and upper layers of the atmosphere. Widespread moderate rain and embedded heavier cores are expected, but we reserved the very heavy amounts for the more unstable near-surface air which advects up into the Keys and along the east coast of the peninsula beginning tonight and continuing tomorrow. Certainly the consensus trend was toward spilling more of the heavy rainfall inland along the east coast on Monday, and we followed this trend given the eventual arrival of stronger surface to 850-mb inflow beginning tonight. WPC QPF was based largely on WPC continuity and the NAM CONUS Nest. ...Southern Plains... This will be a very difficult scenario in which to pin down specific thunderstorm evolution with much confidence. The region near and east of the dryline has been strongly capped the past few days owing to both warm mid level temperatures and marginal moisture depth. Both parameters have improved only a smidge, judging by the SAT - SUN soundings. This evening moisture will back up to the caprock beneath weakly difluent upper flow and very subtle height falls ahead of the western U.S. trough. A few thunderstorm clusters are possible or even likely, but we have less confidence in the scope of this activity. The global models are much more conservative than the hi-res models in this case, and the hi-res may be over-playing the instability and under-playing the inhibition that will likely squash convection during the late evening. On the other hand. If enough cold pool production can take place, then a heftier QPF swath could result, such as near the Red River in some of the hi-res guidance. We played this conservatively in the WPC QPF, using to some degree the GFS and WRF-NMMB. There would seem to be a better chance of scattered coverage of storms on Monday as westerly flow improves a bit and the dryline becomes more SW to NE oriented, increasing convergence over especially NW OK and adjacent KS/TX. ...Western U.S. and Front Range... The western U.S. low will continue to spin down, while occasional convergence and instability will support showers, particularly over eastern Nevada near the low center. Southwest flow atop post-frontal upslope will lend toward scattered low-topped convection from northeast Colorado into adjacent WY/NE both today and tomorrow. We used the NAM CONUS Nest and WRF-NMMB for some of the smaller scale details. Days 2/3... ...Great Basin, Northern/Central Rockies and High Plains... A well-defined closed mid level low continues to be positioned over the Great Basin as seen in the latest GOES-16 WV imagery. However, the models forecast this system to begin to gradually weaken through late Monday and Tuesday. Some of the energy will eject off to the northeast across the High Plains, but by Wednesday there will a new mid level trough and associated closed low moving inland across CA which will allow the remaining energy leftover across the Great Basin to eject northward around the eastern flank of the new CA closed low. Plenty of shower activity will continue across the Great Basin Tuesday and Wednesday given a combination of relatively steep lapse rates, diurnally heating and orographics. Although some of the heaviest shower activity will tend to be on Wednesday across the northern Sierra-Nevada and up over southern/central OR and into areas of the far northern Rockies where upper level forcing will be stronger around the northern flank of the closed mid level low centers and related troughing. Meanwhile, farther to the east across the immediate High Plains region of southeast WY and eastern CO, there should be some late afternoon and evening showers and thunderstorms. This will be aided by modest and relatively moist low level east to southeast flow coupled with strong diurnal heating and arrival of ejecting vort energy from the base of the Great Basin closed low. There is some model signal for a potentially more organized convective cluster Tuesday evening in particular across the CO High Plains, and thus some locally heavy rainfall totals will be possible, but in general much of the region should see relatively modest rainfall totals for the period. A multi-model blend led by the GFS and ECMWF was preferred across the West overall. ...Central and Southern Plains... Shortwave energy tracking out of the Southwest through the period around the base of the Great Basin closed low will advance downstream within deep layer southwest flow and continue to interact with a deepening pool of moisture and instability focused along a slow-moving front settling down across the southern High Plains and stretching northeast toward the mid MS Valley. The result will be several of clusters of heavy showers and thunderstorms which will be aided by precipitable water values between 1.25 and 1.5 inches in vicinity of the front and at least moderate instability. Locally heavy rainfall is expected, and at least a low-end risk of flash flooding exists as a result. The most organized convective potential will be generally from west TX northeast up across central OK and through eastern KS and northwest MO with locally a couple inches of rain possible. Again generally favored a multi-model consensus for QPF. ...Mid and Upper MS Valley east to the OH Valley, Mid-Atlantic and Northeast... Shortwaves embedded within fast mid level flow around the southern edge of the westerlies will again be interacting with a front and a pool of relatively deep moisture and instability along it for several clusters of convection in vicinity of this boundary through the period. The heaviest rainfall threat through Monday night and Tuesday should tend to be from areas of southern IA and northern MO east across portions of central IL and IN which will be a result of stronger vort energy interacting with at least a modest low level jet in vicinity of the front. Some of this will translate downstream across the OH Valley and the northern Mid-Atlantic region, but there should actually be a threat for more concentrated and heavier rainfall Tuesday night and Wednesday from the upper OH Valley east across the central Appalachians, northern Mid-Atlantic and southern New England where stronger upper level jet dynamics are expected to set up. This coupled with surface wave activity along a front and plenty of moisture will favor more organized and stronger convection. Locally a couple of inches of rain can be expected, and some flash flooding will be possible. ...FL/Southeast... An upper low over the eastern Gulf of Mexico coupled with a developing surface reflection is expected to drive a significant amount of tropical moisture off to the northwest across FL and into the interior of the Southeast through the period. While the heaviest rainfall potential may tend to be during the day 1 period, there will be additional heavy showers and thunderstorms with potential for more organized convective bands advancing inland. Locally several inches of additional rain cannot be ruled out. Burke/Orrison Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml