Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 415 AM EDT Thu Apr 19 2018 Prelim Day 1, Day 2 and Day 3 QPF Discussion Valid Apr 19/1200 UTC thru Apr 22/1200 UTC Reference AWIPS Graphics under...Precip Accum - 24hr Day 1 ...Northeast... A respectable amount of synoptic forcing will accompany a shortwave trough moving into the mean larger scale troughing over the Northeast this morning. Height falls, upglide on the 300 K isentropic surface, and frontogenesis just north of the developing coastal low will all support a broad area of rain and snow. Moisture is, however, lacking in an overall sense, and the moist plume will move quickly offshore after 12z. Cold air aloft and the increasingly high April sun angle will support diurnal showers, especially in the higher elevations of NY, PA, but with precipitable water values falling to less than a half inch by midday, this system will not produce a lot of liquid. The more noteworthy impacts will be from accumulating snow in the early morning over parts of NY, PA. See the WPC Heavy Snow Discussion, QPFHSD, and local forecast products for details. WPC QPF was derived from the GFS, UKMET, and SREF, and was supported by some of our in-house ensemble tools. The 12z ECMWF was thought to have been too suppressed southward with its QPF. ...Western U.S... Despite a fairly classic appearance in loops of water vapor imagery, the low entering the West Coast was not very anomalous at only 1 standard deviation from climatology in the 500 mb height field. Moisture is also scant, such that this system is more of a wind producer and will enhance the wildfire potential in the Southwest per Storm Prediction Center and local forecast office products. There will, however, be some organized areas of light to moderate rain and snow, especially in the northern portion of the mid level circulation / the Sierras across the Great Basin, and also where some southerly moisture return meets the difluent upper flow pattern ahead of the trough / West Texas through New Mexico into the western and central Colorado. Given well clustered model solutions, a consensus approach should work well here. We leaned on continuity and the non bias corrected version of our in-house ensemble which is more generous with coverage of measurable precipitation. We also leaned toward the WRF-ARW2 and GFS with respect to convection and elevated convection expected in parts of TX, NM this afternoon and evening. The CAPE axis sets up far to the west, along I-25 per the NAM. Storms firing here may have a difficult time making eastward progress into drier, more stable air, but a few may tend to be more long lived near and south of the Rio Grande within a broader field of moisture pooled along a frontal zone. Days 2/3 ...Pacific Northwest/Northern Rockies... A negatively tilted long wave trough crosses northern WA and adjacent British Columbia during Day 2, reaching the Northern Rockies later in the period. The next short wave in the flow approaches the Pacific Northwest coast late on Day 3. There was generally good model agreement with the evolution of the mid level systems, so the WPC QPF was based on a blend of the 00z ECMWF/00z GFS. Day 2... A surface cold front associated with the negatively tilted long wave trough crossing northern WA during Day 2 focuses 0.75 inch precipitable water air in the 25 knot low level flow on the Olympic Mountains and northern WA Cascades. While the best lift and thrust of moisture ahead of the front are expected over Vancouver Island and southwest British Columbia, local 0.50 inch qpf amounts were placed over higher terrain of WA. Day 3... The short wave energy crosses the northern Rockies early on Day 3, and the lift with the short wave taps some of the moisture that survives from from the Pacific plume. The combination of moisture and upslope flow is expected to produce local 0.25 inch qpf amounts over the Rocky Mountain Front Range. Further west, lift ahead of an approaching short wave should be strong enough to focus the 0.50 inch precipitable water air on the northern and central WA Cascades, mainly between 22/06z and 22/12z. Local 0.25 inch qpf amounts were placed over the northern WA Cascades ahead of the strong short wave. ...Great Basin/Rockies/Central Pains... A closed mid level low over UT/AZ early on Day 2 reaches eastern CO/eastern NM by the end of the period. Lift with the closed mid level low and upslope flow is expected to make the most of the moisture in the column to produce locally heavy QPF in the higher terrain, mainly during Day 2. The 00z ECMWF seems to be a tad on slow side with the mid level system, so the WPC QPF was based more closely on the faster 00z NAM/GFS. Day 2... As the closed mid level low trundles across the Four Corners area during Day 2, a 25/35 knot low level south southeast flow transports 0.75 inch precipitable water air into the western High Plains from western NE into western KS and the Panhandle of OK, mainly between 20/18z and 21/06z. Model soundings showed marginal to moderate instability in the moisture plume extending from western KS and eastern CO into western OK during this time. The resulting convection is expected to form on the terrain and move into the Plains after 21/00z. There was a good model signal for 0.50/1.00 inch qpf amounts extending across western NE into western KS, and this formed the basis for the WPC QPF. Further west across CO, the low level south southeast flow veers as the mid level system approaches, funneling some of the moisture from the Plains into the higher terrain. The upslope flow focuses the moisture on the higher terrain, resulting in a qpf axis of 1.25 inches on the Front Range in CO, with lesser amounts (between 0.50/1.00 inches) over the CO Rockies into the San Juan Range in southwest CO and nearby northern NM. Finally, moisture along the Wasatch Mountains and Uintas in the upslope flow is expected to produce local 0.50 inch qpf amounts. Heavy snowfall is possible on Day 2 across the higher terrain of the Rockies. Please refer to the most recent suite of WPC Winter Weather products for more information. Day 3... Residual moisture in the upslope flow as the mid level closed low passes over central and eastern CO during the first part of Day 3 should be sufficient to produce 0.10/0.25 inches of qpf over the CO Rockies. ...Southern Plains into the Lower MS Valley... A closed mid level low over eastern CO/eastern NM at the start of Day 3 tracks toward OK/AR by the end of the period. An inverted trough associated with the mid level system becomes the focus for deepening moisture and instability, which feed convection that may produce heavy to excessive rainfall, especially across the Lower MS Valley. The 00z ECMWF continues that model's slower motion with the mid level system and its attendant surface trough, so the WPC QPF was based more closely on the 00z GFS, with some 00z NAM thrown in as well. Day 2... Moisture increases across OK and portions of central and western OK during Day 2 ahead of a closed mid level low crossing the Four Corners area. A 30/40 knot low level south southeast flow transports 0.75/1.00 inch precipitable water over western and central OK and the TX Panhandle. The developing convection may become organized by lift associated with the closed mid level low, and there should be enough mid level flow to keep the convection moving east. There was a good signal from the 00z GFS/00z NAM for an area of 0.25/0.50 inches of qpf over this area, with local 1.00 inch qpf amounts possible, Day 3... Surface low pressure that forms over west TX ahead of the closed mid level system over the Rockies early on Day 3 weakens as it tracks across east TX during the first half of the period. The trough becomes the focus for increasing moisture, as a 30 knot low level southerly flow transports 1.50 inch precipitable water air into western and central LA/AR after 22/00z. Model soundings showed generally marginal instability along and ahead of the inverted trough (due in part to the time of day), but divergence in the left front quadrant of a 110 knot jet streak crossing TX into LA after 22/00z could allow the convection to become better organized, partially compensating for the marginal instability. As mentioned earlier, the 00z ECMWF appears to be too slow with the surface and mid level systems, so the WPC QPF was based more closely on the 00z GFS/00z NAM (though it is possible that the 00z NAM may be a bit too deep, at least when compared to other 00z guidance). As the surface and mid level system move eastward, a large area of 1.00/1.50 inches of qpf was stretched from eastern OK/northeast TX into southern AR/northern LA and far west central MS. For the most part, three hour flash flood guidance are fairly high (especially over OK/northeast TX, where values are generally above 3.00 inches), so a Marginal Risk was placed over this area to account for the convection. Given the 00z GFS placement of the highest qpf amounts over northern LA, a Slight Risk wa placed here on Day 3. The National Water Model shows the greatest stream flow increases here, suggesting high flow before 22/12z. The Slight was placed over far southern AR and far west central MS to account for 00z NAM placement of the highest qpf amounts. ...New England... Short wave energy on the back side of a closed mid level low heading for the Canadian Maritimes during Day 2 will produce upslope flow over northern New England. The upslope is expected to produce qpf across the higher terrain. There was generally good model agreement with the overall synoptic setup, so the WPC QPF was based on a multi model blend, with some 00z NBM used to better delineate higher terrain qpf maxima. The boundary layer north northwest flow behind a closed mid level low tracking toward the Canadian Maritimes during Day 2 lifts cold and moist air up over the higher terrain extending from northeast NY into western ME. Over the favored upslope areas, local 0.25 inch qpf amounts were placed, with the highest amounts over the Green Mountains in VT and the northern White Mountains in NH. ...FL... Surface high pressure off the East Coast produces an easterly flow that transports moisture and instability along a weakening boundary during Days 2 and 3. Convection that develops is expected to produce mainly light qpf amounts over land each day. Since there was good model agreement with the overall synoptic setup, the WPC QPF was based on a multi model blend. Day 2... A weakening boundary over central Fl during Day 2 focuses moisture and instability in the easterly low level flow south of surface high pressure over the East Coast. The low level flow transports 1.25 inch precipitable water air and marginal instability over east central FL, where there was a good model signal for 0.25 inches of qpf. Locally higher amounts are possible along the coast in the low level convergent flow. Day 3... The synoptic setup is essentially the same as Day 2, though there could be more moisture in the column, as model soundings are showing local 1.50 inch precipitable water air along the east central FL coast on day 3. The mainly diurnally driven convection could produce local 1.00 inch qpf amounts here, as the low level easterly flow remains convergent. Burke/Hayes Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml