Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 453 AM EDT Fri Mar 16 2018 Prelim Day 1, Day 2 and Day 3 QPF Discussion Valid Mar 16/1200 UTC thru Mar 19/1200 UTC Reference AWIPS Graphics under...Precip Accum - 24hr Day 1 ...Overview... A compact, fairly vigorous low will drift east across the Central Plains, bringing rain, ice, and snow. The system will begin to weaken by Friday night as it has limited real estate with the large, stable trough still occupying the eastern United States. Within the eastern trough over New England and parts of OH/PA/WV, continued northwesterly flow and a sufficiently moist boundary layer / steep lapse rates will yield additional rain/snow showers to little effect in terms of areal average liquid equivalent. Meanwhile out West a 540 decameter closed low - not particularly anomalous - will lumber its way inland through California, Oregon, and the western Great Basin. ...Western U.S... Sub-tropical moisture is suppressed, but the trough along the West Coast will make the most of PW values peaking around a 0.50-0.60 inches, with strong 700 mb flow directed orthoginally into the Sierras. At lower levels, 850 mb, the models forecast a slight increase of onshore flow wind speeds into Southern California by late afternoon and evening, along with moisture convergence / a slight spike to PW values. This area will sit favorably along the southern periphery of strong height falls associated with the upper low, yielding a combination of increased westerly flow and some deep layer ascent. Thus, we leaned toward heavier guidance such as the 00z WRF-ARW over Southern California. Elsewhere throughout the West, cold air aloft and the broad scale cyclonic flow will support a showery regime over much of Oregon, and some semblance of a renewed frontal zone / secondary Pacific front behind the one which entered the Great Plains - will help to focus precipitation in Nevada and Idaho. WPC QPF here was derived from a blend of the 00z HREF Mean, WRF-ARW, our in-house ensemble summarizing the larger scale guidance, and 20 percent continuity from the previous forecast. ...Central / Eastern U.S... Compact cyclogenesis will favor a swath of at least half inch to one inch areal average liquid equivalent, and locally greater amounts over parts of NE/SD/IA, and especially focused in the daytime hours. Models are relatively similar, but they do vary in the orientation of the precipitation axis. Models appeared a little bit split on whether to favor the 850-700 mb ascent which fans out northward over time, or to favor the 925-850 mb convergence that will occur farther south near the surface frontal zone. The WPC forecast smooths out this choice a bit, but with a seeming continued shift southward in the guidance over NE/IA, some tweaks are possible in the final 0830z WPC forecast which could reduce precipitation amounts in far southeast SD and northwest IA. Will have to monitor observational trends. Otherwise, we attempted to keep continuity while incorporating the 00z WRF-ARW and HREF. This helped latch onto some of the trend over NE/IA, and we also favored the GFS which was more generous with some of the shallow convection that is likely to take place under the compact, stacked system later today in KS/NE/MO. From the Plains system heading northwest, vorticity pinches off and a definable shortwave is expected to lift through eastern Montana, resulting in a local maximum of precipitation there. Farther east, warm advection and upglide on the 295-300 K surfaces will support rain arcing out through the lower Ohio Valley / Tennessee Valley and lower Mississippi Valley. A consistent signal has been for a more concentrated rainfall over parts of LA/MS, supported by a southern stream jet streak and proximity to deeper moisture seen on the evening soundings over south Texas. The SREF 6-hourly rainfall probabilities were useful, as was the previous WPC forecast which seemed to have a good handle. Days 2/3... ...Pacific Northwest/Northern Rockies... A closed mid level low over the Pacific Northwest on day 2 opens up into a long wave trough that crosses the Northern Rockies during day 3. The combination of lift associated with the mid level low and moisture ahead of it produces QPF each day. There was generally good model agreement with the evolution of the mid level system, so the WPC QPF was based on a blend of the 00z ECMWF/GFS. Day 2... A closed mid level low over northern CA/OR/northwest NV early on day 2 weakens as it becomes absorbed by a larger long wave trough extending over much of the Intermountain West. While the moisture associated with the mid level system is rather sparse, the combination of upslope flow and lift with the closed low is expected to produce an axis of 0.25 inches of qpf over the central OR Cascades. The mid level system weakens as it crosses the Northern Rockies during the second half of the period, but upslope flow focuses the available moisture to produce a large area of 0.50/0.75 inches of qpf over the Bitterroot and Sawtooth ranges in UT. Day 3... A second closed mid level low crosses the Pacific Northwest coast late on day 2 into early day 3. There is a bit more Pacific moisture with this closed mid level low, so the areal coverage of QPF was expanded across much of the OR Coastal Range, as well as the OR Cascades. Upslope flow ahead of the mid level system, combined with marginal instability (due at least in part to steepening lapse rates), is expected to produce areas of 0.25 inches of qpf over the northern and central Bitterroot Range in ID. Locally heavy snowfall is possible over the Northern Rockies, mainly on day 2. Please refer to the most recent suite of WPC Winter Weather products for more information. ...Great Basin/Southern Rockies/Southwest... Short wave energy over Southern CA on day 2 evolves into a long wave trough that crosses the Southern Rockies and Southwest states during day 3. The long wave trough aids in spinning up low pressure over southeast CO during day 3. There was generally good model agreement with the overall synoptic setup, so the WPC QPF was based on a blend of the 00z ECMWF/GFS. Day 2... As the short wave energy stretches into a long wave trough over the Southwest states on day 2, the best lift over the Great Basin and Southwest states occurs between 18/06z and 18/12z. Increasing low level moisture on a southerly flow introduces 0.50 inch precipitable water across the Wasatch Range in UT, as well as the Mogollon Rim in AZ. The combination of moisture, lift and marginal instability is expected to produce axes of 0.25/0.50 inches of qpf across the aforementioned locations. Day 3... The long wave trough crosses the Southern Rockies and NM during day 3. Moisture in the low level flow becomes focused on the highest terrain in CO as the flow becomes upslope. There was a multi model signal for a large area of 0.25/0.50 inches of qpf over the CO Rockies, with local 0.75 inch amounts expected across the Front Range and over the higher terrain of the San Juan Mountains. Locally heavy snowfall is possible over the Southern Rockies, mainly on day 3. Please refer to the most recent suite of WPC Winter Weather products for more information. ...Northern and Central Plains... As a long wave trough crosses the Northern Plains on day 3, moisture is drawn northward ahead of the mid level system on an increasing low level southeast flow along an inverted surface trough. The resultant QPF could be locally heavy over the Central Plains. There was generally good model agreement with the evolution of the synoptic scale systems, so the WPC QPF was based on a blend of the 00z ECMWF/GFS. A weakening long wave trough tracks from the Northern Rockies to the Northern Plains during day 3. Ahead of the mid level system, a 30 knot low level southeast flow transports 0.75 inch precipitable water across central KS into western NE, along an inverted surface trough extending across NE in central NS/SD. Model soundings showed marginal instability along the inverted trough, which wraps around a developing surface low moving from southeast CO into the Southern Plains. The combination of moisture and instability feeds low topped convection that produces an axis of 0.75 to 1.50 inches of qpf stretching from north central KS into west central NE. It should be noted that the 00z GFS had qpf amounts in excess of 2.00 inches over portions of western NE, but most other 00z solutions has values closer to the WPC QPF amounts. The highest qpf amounts appear as though they will occur over the higher flash flood values of the Sandhills, and some of the qpf will fall in the form of snow. For now, no excessive area was placed over west central NE, but if later runs consistently place qpf over lower flash flood guidance values, a Marginal Risk area could be needed in later forecasts. ...Southern Plains/Gulf Coast states/Southeast A developing long wave trough over the Southwest states on day 2 tracks across the Southern Plains during day 3, as it closes off in the mid levels. Surface low pressure forming near the closing mid level system focuses deep moisture and instability along a frontal boundary extending from the Southern Plains into the Southeast states during day 3, resulting in heavy to excessive rainfall. The 00z NAM is likely too far north with its QPF placement in association with the front. With this in mind, the WPC QPF was based more closely on a 00z ECMWF/GFS blend. Day 2... As short wave energy crossing Southern CA early on day 2 develops into a long wave trough that crosses western AZ, the lift with the mid level system aids in spinning up surface low pressure over southeast CO. A frontal boundary associated with the surface low extends across northern and eastern TX across LA late in the period, and the front becomes the focus for 1.50 inch precipitable water air and moderate instability. The moisture and instability are expected to feed convection that produces axes of 0.25 inches of qpf across northeast TX and southern LA into southern MS. Day 3... The long wave trough evolves into a closed mid level low over northern OK during day 3, as surface low pressure tracks across the same area. The frontal boundary extending from the low across far northeast TX/northern LA into central MS/AL is once again expected to focus deep moisture (with precipitable water values of 1.50/1.75 inches, which is about two standard deviations above the mean) and moderate instability to feed convection, mainly after 19/00z. As mentioned earlier, the 00z NAM appears to be too far north with its frontal position and its qpf maximum. The 00z GFS/ECMWF were closer with the position of the front, and the axis of maximum qpf was placed close to the 00z GFS, as it appears to be closest to the best instability extending north from the Gulf of Mexico. While there was a good consensus with the placement of the maximum qpf between the 00z GFS/ECMWF, the 00z ECMWF was lighter, which seems too light given the amount of moisture available. Because of this, the WPC QPF was closer to the 00z GFS amounts. The WPC QPF amounts are lower than the three hour flash flood guidance values, but with rivers and streams still in flood, and the National Water Model showing a significant reaction in the streamflow, a Slight Risk area was extended from southeast AR/northeast LA across central MS into central AL, where training could result in locally higher qpf amounts. ...OH Valley/Mid Atlantic... A closed mid level low over the western OH Valley early on day 2 weakens into a short wave that crosses the Mid Atlantic coast by the end of the period. Moisture and marginal instability becomes focused on a surface low associated with the mid level system, producing locally heavy rainfall. There was generally good model agreement with the overall synoptic setup, so the WPC QPF was based on a blend of the 00z ECMWF/GFS. As a closed mid level low over western IL tracks across the OH Valley across the Mid Atlantic states during day 2, it weakens as it approaches the coast. Its attendant surface low tracks along the same area, with the best low to mid level lift occurring prior to 18/00z. A 40 knot low level southwest flow funnels 0.75 inch precipitable water and marginal instability across IN/OH into KY/WV and southern VA northern NC. Low topped convection ahead of the surface and mid level systems is expected to produce axes of 0.25 inches of qpf across central OH into central WV, as well as southwest VA into east central NC. Burke/Hayes Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml