Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 508 PM EDT Wed May 16 2018 Final Day 1, Day 2 and Day 3 QPF Discussion Valid May 17/0000 UTC thru May 20/0000 UTC Reference AWIPS Graphics under...Precip Accum - 24hr Day 1... ---Eastern U.S.---- There is not expected to be any significant changes to the very wet pattern affecting large portions of the eastern U.S. A very large area of much above average pw values---1.5 to 2.5+ standard deviations above the mean will persist along and to the east of the elongated mean trof stretching from the OH Valley---into the TN Valley and central to eastern Gulf Coast region. Widespread showery conditions will persist from Florida--northward through the Southeast---Southern to Central Appalachians---Mid-Atlantic into southern NY state and southern New England. The href mean and in house hi-res mean qpf was used primarily for qpf details day 1 to mitigate the typical run to run and model to model qpf detail differences. The combination of the much above average pw values and broadly difluent upper flow to the east of this mean trof will support widespread moderate to heavy totals across these areas. Focus areas for the heaviest precip likely through the Southern to Central Appalachians from northeast GA---the Upstate of SC---western NC---western VA into eastern WV where uvvs will be enhanced by topography. The persistent west to east oriented stationary front between the OH and TN valley---into the Mid-Atlantic will also help focus a potential heavy precip axis through the Mid-Atlantic into far southern NY state and coastal southern New England. Excessive rainfall wise---a broad marginal risk area is maintained across these regions---with slight risk areas along and north of the stationary front from the Central Appalachians into the Mid-Atlantic and over the Southern Appalachians in the vicinity of the above mentioned precipitation maxima. ...Lower MS Valley into the Central Gulf coastal region... Height fall expected to drop southeastward on the back side of the mean trof position stretching from the OH-TN Valleys into the Central Gulf coastal region. PW values are not as anomalous in this area as points to the east of the above mentioned mean trof position---generally around 1 standard deviation above the mean. Convection enhancing over the Southern Plains this afternoon ahead of these height falls expected to weaken by the beginning of the upcoming day 1 period as they push toward the Lower MS Valley. This activity will likely re-ignite later day 1--especially Thursday afternoon over the Lower MS Valley-Central Gulf coast region. There is a lot of model qpf spread here---leading to low confidence. Moderate areal average amounts depicted with the likelihood of locally heavier totals where convection does become most organized. ...Northern California...Great Basin... Models are similar day 1 with the eastward push of the north central California closed low moving into the Great Basin. Showers will remain active to the east and northeast of this closed low from northern California---eastern Oregon---northwest NV. This will support moderate to isolated heavy precip totals across these areas where pw values are expected to remain much above average---2 to 2.5 standard deviations above the mean. No changes planned to the marginal risk area depicted across these areas to the north and northeast of this closed low. ...Northern Rockies... A farther northeast closed low will be moving slowly northward from the Northern Rockies into the northern High Plains this period. Shower activity likely to continue to the north and northwest of this system---enhanced by developing north northeasterly upslope flow into the northern Rockies. Moderate to heavy totals likely from northern Idaho into northwest MT. ...Northern Plains into the Upper MS Valley... Shortwave energy pushing to the northeast of the Northern Rockies closed low will enhance uvvs along and north of the west to east oriented front forecast to lie across the Northern Plains into the Upper MS Valley. The best chance for shower development along and north of this boundary will be Thursday afternoon ---with this activity likely to enhance further after the day 1 time period into the beginning of the day 2 time period. For the day 1 period---the timing of the convective development will likely keep the areal extent of potentially heavy rain fairly small---with an expanding area likely day 2 heading into convective max time early Friday morning. Days 2/3... ...Pacific Northwest/Great Basin/Northern Rockies... A long wave trough making its way across the Intermountain West during Days 2 and 3 provides synoptic scale ascent for convection each day. Deeper moisture and better instability over the northern Rockies eastward into the Northern Plains could result in heavy to excessive rainfall during Day 2, while upslope flow south of a frontal boundary crossing the region on Day 3 could focus convection for locally heavy rainfall. There was generally good model agreement with the synoptic scale features, so the WPC QPF was based on a blend of the 12Z ECMWF/GFS/NAM. As the long wave trough moves across the Great Basin and Rockies during Day 2, and surface high pressure builds south from western Canada, a low level southeast flow becomes focused over western Montana and parts of northern Wyoming. Increasing low level moisture will help boost the static stability over the region, with MUCAPE values in excess of 500 J/KG in these areas by early in the Day 2 period. The flow associated with the long wave trough keeps precipitable water values between 0.75/1.00 inches which is expected to produce an axis of 0.75/1.25 inches of precipitation from far northern ID eastward across the Rocky Mountain Front Range in MT. Three hour flash flood guidance values remained as low as an inch, and the National Weather Model showed high stream flows, so a Marginal Risk was placed here for Day 2. The long wave trough continues to move slowly across the Rockies during Day 3, briefly spawning a closed mid level low over the Northern Rockies. The best instability is expected to stretch from southern ID into eastern WY, and local 0.50 inch QPF amounts are possible over the higher terrain. ...Central and Northern Plains... Moisture and instability streaming north on an increasing low level flow will become focused on a frontal boundary extending from the CO plains into the Upper MS Valley during Day 2, producing convection with heavy to locally excessive rainfall. As the frontal boundary changes orientation, the focus for the instability and moisture extends from the western High Plains into the Upper MS Valley and far western Upper Great Lakes. While there was generally good agreement with respect to the placement of synoptic scale features, there are some differences in the placement of the higher QPF amounts. There was a convergence among the NCEP global models and hi resolution models in the placement of the axis of heaviest rainfall. As a result, the WPC QPF was based on a blend of the 12Z ECMWF/GFS/ARW. Day 2... A 30 to 40 knot low level southerly flow will transport an airmass with precipitable water values around 1.25 inch across the Central and Northern Plains, ahead of a long wave trough moving slowly across the Rockies. Model soundings showed a large reservoir of 1000+ J/KG of MUCAPE available much of the afternoon/night. Convection is expected to initiate in the axis of moderate to strong instability extending across eastern CO/western and central NE, where local 1.50 inch QPF amounts were placed. Both the 12Z GFS/NAM/ARW showed local QPF amounts in excess of 2.00 inches along this axis, with some convergence on the placement of the heaviest rainfall being along a northeast to southwest oriented axis over parts of North Dakota extending back into western South Dakota. Further north across the Northern Plains, the moisture and instability becomes focused on a frontal boundary extending from central MT into ND and western MN after 18/00z. The signal for a cluster of storms/possible MCS were still pretty strong in the 12Z NCEP global models and hi resolution ARW core with spot maximum amounts now running 2 to 4 inches. Since three hour flash flood guidance values remain as low as 1.00/1.50 inches along the ND/MN border, and the National Water Model showed high stream flows before 19/00Z, some minor adjustments were made to the slight risk area and the Marginal Risk area that surrounded it...but the changes did not reflect a fundamental change in the previous forecast reasoning. As the long wave trough moves slowly eastward during Day 3, the axis of deeper moisture and instability extends from western NE across eastern SD into northwest MN, along and southeast of the frontal boundary. Moderate to strong instability remains east of the front, which should aid developing convection track northeast. There was a strong model signal for an axis if 1.00/1.50 inches of QPF over the aforementioned areas, and based on three hour flash flood guidance values, a Marginal Risk was placed here on Day 3. The exception is over far western NE/far southeast WY, where there was a stronger model signal for 2.00/2.50 inches of QPF with the NCEP runs still being insistent on amounts locally exceeding 4 inches. Was very suspicious of their solutions in this part of the country when 6 hourly amounts were 3+ inches during the overnight hours considering how unusual that would be at that time of day. Certainly, the upslope flow will continue to feed moisture from the plains into the upslope region, and there could well be some elevated convection, so certainly continued to forecast rainfall in the area. but we cut back the QPF there for those reasons. There was no reason to make many modifications to the slight risk, other than expanding the slight risk into the upslope region of the Cheyenne Ride. ...TN/OH Valleys into the Mid Atlantic/Northeast... Tropical moisture from the Gulf Coast states and the Southeast becomes focused on a slow moving frontal boundary extending across the TN/OH Valleys into Mid Atlantic during Day 3, moving slowly into the Northeast states during Day 3. Deep moisture and instability will feed convection that produces excessive rainfall each day. There are still some differences in the placement of the frontal boundary (and its associated instability), especially during Day 2. The 12Z EMCWF/GFS seemed to have the best handle on the frontal position. Thus the WPC QPF was based on a multi model blend based on discussions with the WPC model diagnostician. Three hour flash flood guidance has been lowering during the past couple of day due to rounds of convection. The National Water Model showed stream flows running high in these areas, so a Slight Risk was placed in these areas for Day 3. ...Southeast/FL... Deep tropical moisture associated with a mid level and surface system tandem combines with in situ instability to produce convection capable of heavy to locally excessive rainfall each day, with the best chance of excessive rainfall over the east coast of FL. There was generally good model agreement with the placement of the synoptic scale features, as well as the higher QPF amounts, so the WPC QPF was based on a multi model blend. Oravec/Bann Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml