Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 434 PM EDT Mon Jun 04 2018 Prelim Day 1, Day 2 and Day 3 QPF Discussion Valid Jun 05/0000 UTC thru Jun 08/0000 UTC Reference AWIPS Graphics under...Precip Accum - 24hr ...Southern U.S... From this afternoon through Tuesday the predicted rainfall over the CONUS is low on coverage and not especially heavy on average. One of the heavier stripes of expected rainfall extends from near the Red River down to New Orleans / Biloxi, straddling a stationary frontal zone. Low level moisture will pool and gradually deepen near the front, and a mid level trough slicing through the mean larger scale ridge position should support better organized clusters of thunderstorms by late tonight into Tuesday. In the early hours of the forecast, however, we were somewhat pessimistic as to how much convective coverage would be realized. The MCS that rolled out of the Texas Hill Country this morning had dissipated, and morning RAOBS in the southern Plains and lower Mississippi Valley revealed a lot of very warm and dry mid level air with weak to mid range lapse rates, as well as shallow low level moisture. WPC sided with the HRRR and WRF-NMMB which suggest if any organized afternoon convection is to form it is more likely to occur with eastward extent, ahead of the remnant MCV and toward the cooler - roughly 8 C at 700 mb - mid level temperatures in far east Texas and Louisiana. Much of the remainder of central Texas up through Oklahoma may remain capped, although with strong heating and holding onto some continuity, we do indicate some widely scattered activity. Expect coverage to increase overnight, but it may wait until later in the night over OK/TX, as the low level jet veers to induce stronger warm advection / more steeply crossing the mid level isotherms. Even still, the core of the low level jet will be located farther west such that flow speeds in the area of deep forcing near the mid level trough will likely remain light, less than 25 knots at 850 mb. Wherever activity does develop, however, there will be potential for very slow movement initially, and some locally heavy rainfall amounts. Given the limiting thermodynamic factors listed above, WPC leaned as far north and east as possible from late tonight into Tuesday, using a majority WRF-NMMB and NAM CONUS Nest - which line up well with the expected north side of the synoptic front per the NAM. This approach resulted in lesser QPF over Texas and greater QPF downstream into Louisiana and the central Gulf Coast. ...Northern Plains... A sharp, but low amplitude shortwave trough will eject through the northern Rockies this evening and tonight. Primary height falls will occur up in Canada, but some deep layer forcing and a generally difluent jet structure will coincide with an instability axis over parts of central and eastern Montana, leading to at least scattered thunderstorms. Given the flow strength, cells will be moving quickly, so there is relatively little risk of excessive rainfall in this region, but most-unstable CAPE values above 1000 J/kg and PW values just shy of an inch open up the possibility of a quick accumulation of an inch or more in some isolated locations, owing to brief training or cell mergers. Guidance is all pretty agreeable as to the areal average rainfall. Downstream the event will increasingly become dependent on warm advection overnight in North Dakota, and as the more surface-based activity diminishes out of Montana, there may be a local minimum of coverage / intensity in western ND, picking up farther east with stronger warm advection. This regime eventually leads to likely MCS development the following night, Tuesday night, downstream into eastern ND per the Day 2 forecast. ...Great Lakes / Northeast / Mid Atlantic... The main warm conveyor and deep moisture will remain offshore ahead of east coast troughing. Continued height falls, however, working with sufficient residual moisture, will lead to a rainy period for northern New England, and a sharp shortwave trough diving through the lower Lakes into the Ohio Valley will bring some showery / thundery weather as well. The more stratiform area over New England should be well handled by a consensus QPF approach. From the Lakes into the Ohio Valley the synoptic forcing per the global models supported leaning on QPF from the ECMWF and the WRF-NMMB (which was preferred in other regions as well). Day 2 and 3... ...Northern Rockies to the Northern and Central Plains... In general, the Day 2-3 period in the Central CONUS will be marked by a broad mid-upper level ridge, with the ridge axis extending up the Plains roughly along 100W longitude. Much of the rainfall in this period (from Tuesday Night through Thursday) should be positioned near or north of the I-80 corridor, around the northern periphery of the elevated mixed layer (generally outlined by the +12C isotherm at 700mb). Some models wanted to extend some light QPF further down the Plains, even into West Texas, likely the result of some isolated convection. While this can't be ruled out, the environment should generally be hostile and there was not sufficient confidence in the precision of any localized QPF maxes to include rainfall further south than central Kansas. Some of the more focused heavy rainfall will likely be from North Dakota into Minnesota from Tuesday Night into early Wednesday, as a stronger cold front pushes into a highly unstable air mass. This appears to be a favorable setup for MCS development, and it should drop increasingly to the southeast and south around the periphery of the ridge with time into Wednesday Night and Thursday -- either as a continuous cluster of thunderstorms, or with new convection regenerating on outflow. This will be a favored area for heavy rain given the likelihood of organized convective clusters (strong instability and wind shear), but convection should also tend to be forward propagating. Therefore, a Marginal Risk was maintained in this area for Day 2, and introduced into southern Minnesota and Iowa on Day 3. The timing of the QPF maximum was a blend of the timing of the GFS and ECMWF. However, the QPF was not necessarily directly blended given the differences in timing. Went close to the ECMWF and internal bias-corrected pseudo-ensemble for the QPF magnitude, but shifted it slightly faster. Elsewhere in the region, QPF generally followed a broader model blend, with a slightly greater weight placed on the 12Z ECMWF. ...Gulf Coast... A broad, digging trough should push into the Gulf Coast region during the Day 2-3 period. And there is likely to be a (relatively, for this region in the warm season) focused ribbon of rainfall ahead of that trough in the plume of higher PWATs. Model QPFs showed greater disagreement on the 00Z cycle, but have converged more favorably on the 12Z cycle. Therefore, a blend of the GFS and ECMWF was generally used, with a hi-res model mean also incorporated early on the Day 2 period. The resulting QPF was adjusted slightly to focus rainfall more offshore during the overnight periods, and over land during the daytime periods, as is typical with the diurnal cycle around the Gulf in the warm season. A Marginal Risk was introduced for Louisiana and adjacent far southern Mississippi and Alabama on Day 2. Hi-res models showed potential for 1-2 in/hr rain rates with the 12Z HREF showing small areas of around 10% probability of FFG exceedance. Burke/Lamers Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml