Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 612 PM EDT Tue May 01 2018 Final Day 1, Day 2 and Day 3 QPF Discussion Valid May 02/0000 UTC thru May 05/0000 UTC Reference AWIPS Graphics under...Precip Accum - 24hr Day 1 ...Central Plains into the Mid/Upper MS Valley and Great Lakes... Convection will expand in coverage this afternoon/evening across the Central Plains and MS Valley. In general this convection will be progressive in nature but there are two areas in which locally heavier rainfall and a greater flash flood risk may exist. The southern area is from northern KS into northwest MO and far southeast NE, and will account for the potential of slow moving and/or training supercells that should develop this afternoon and evening. This activity may train along a strengthening stationary front in the area, and will have the benefit of large CAPE and strong 850 mb moisture transport into the boundary. Limiting factors here are the displacement from the better synoptic forcing and the presence of a cap. Thus, it is unclear if this activity will indeed become organized through the evening...or if it will tend to remain more isolated in nature and weaken after sunset. Generally seeing a mixed signal amongst the 12z HREF members...although recent runs of the HRRR and Experimental HRRR are more aggressive with QPF and organization. Confidence is only medium...but given the impressive 850 mb moisture transport and presence of a stationary boundary...tend to think convection will indeed become more organized in nature. Thus we leaned the WPC QPF towards the more aggressive solutions, such as the Experimental HRRR and HRW ARW. The consensus amongst solutions is for an axis that is farther south than our previous forecast. The northern area across northeast IA into far southeast MN and southern WI will have the benefit of stronger synoptic forcing in the right entrance region of a strengthening upper jet. Limiting factors here will be CAPE and the potential for the convection farther south to rob some of the inflow. However, we think CAPE will be sufficient enough for convection to form, and 850 mb moisture transport vectors suggest enough of a separation from the southern activity to potentially limit the aforementioned robbing of inflow. Thus, while not a sure bet, it looks pretty good for a second convective rainfall maximum near the MN/IA border into southern WI tonight, with some training possible along the front in the area. We have noted a southward trend in this axis in recent HRRR runs. Thus WPC shifted in this direction as well, with our forecast weighted towards the 16z HRRR, with some of the HRW ARW and ARW2 blended in as well. Convection should be on a downward trend Wednesday morning, but we anticipate another round of convection Wednesday afternoon as height falls begin to overtake the dryline over the Central Plains. Some model differences on exactly where this convection initiates during the afternoon hours. The 00z ECMWF and 12z UKMET were in good agreement focusing on central KS. Given some consistency with this location, WPC QPF is weighted towards these solutions Wednesday afternoon. The high res models can sometimes struggle with this more synoptically driven initiation at longer lead times. Thus, leaning towards the global UKMET/ECMWF is sometimes better in these situations, with the high res becoming more useful as we get closer to the event and/or convection is being driven by more mesoscale processes. ...Western U.S.... The amplifying upper trough will generate a broad area of generally light-moderate QPF -- which given the degree of forcing (broad area of favorable upper level difluence) -- will not be linked exclusively to the elevated terrain. Some locally heavy rain is possible across the Southwest, although limited instability should generally cap the rates. WPC generally followed a multi model blend weighted towards the 12z HREF mean across this area. Days 2/3... ...Western United States... At the beginning of the forecast period, the closed low over the Four Corners region, starts to lift northeast with increased elongation, leaving Southeastern WY/Northern Colorado Front Range north of the 850-7H lows. Additional, strong upper level evacuation with strong divergence/diffluence enhances the strengthening low level flow as well as western branch of warm/moist conveyor belt. A slight shift of guidance toward the west supports greatest ascent coincident with the elevation/Front Range to maximize precipitation. The 12z GFS may be a bit over done with given a bit of increased warm air relative to the NAM/ECMWF but WPC QPF matches quite well to the location of a multi-model and ensemble mean consensus. QPF maxima though around 2.0" for 24hr ending at 00z Friday, likely in the form of snow about 7000 ft and above. Please refer to QPFHSD and additional WPC graphical weather products, which are being produced for the 18z/00z packages today and tomorrow, May 1 and 2 past the April 30th deadline. Note: there will NOT be a 06/12z overnight update. ...Rooftop of Maine...Day 2... Zonal flow will be well established across Southeast Canada, but the weak shortwave enhanced by convection in the Midwest later this evening will quickly track through Southeast Canada, clipping the Rooftop of Maine between 00-06z on Thurs. Sufficient deep moisture transport and mild convergence ahead of the wave (near weak surface low along frontal boundary) will support modest intensity with areal QPF values in the .5" range. A blend of the GFS/NAM and ECMWF were used to place the axis of the QPF, but tempered the GFS/NAM greater QPF values to something more reasonable without significant deep moisture (about 1.25") limited instability (less than 200 J/Kg) and more importantly duration of greatest ascent on a weakening small wave. ...Central Plains to southern Great Lakes to Northeast... At the start of the Day 2 period: 00z Thurs, strong moist return flow out of the western Gulf of Mexico surges north ahead of the emerging mid-level large scale trof out of the Western US with well established dry line from the Big Bend Region to W OK/TX panhandle to the Developing surface low in N Central KS...before turning northeast across the Midwest into the southern Great Lakes into the Northeast. Weak precursory shortwave emerges into the Plains late Wed sparking deep convection in the warm sector across KS. By 00-06z, convection is expected to further expand north across SE NEB into IA with the main forcing lifting northeast...some cells may hang up across this region waiting for the best forcing, but of greater concern, this area is generally aligned with convection expected tonight on Day 1 which is likely to have wet or even saturated soils in places reducing FFG values and increasing this secondary bout of potentially excessive rainfall. As such a Slight Risk was introduced from NE KS to south central IA where best hi-res convective guidance as well as higher resolution 12z GFS/ECMWF support greatest overlap. A Marginal Risk extends Southwest into central KS to account for potential for supercells capable of producing flash flooding rains/rates over 2"/hr that develop along the dry line and are feed by ample low level moisture supporting TPWs up to 1.5". This weak southern stream shortwave will shift eastward into the confluent deep layered flow over the Midwest, southern Great Lakes. By early Thursday, northern stream influence will start to take over as the base of a slightly amplified wave crosses the northern Great Lakes providing modest divergence aloft to support a weak surface wave. Loss of better lapse rates and instability will reduce the depth of the convection and broader shield/moderate rain rates will track through the southern Great Lakes Thursday evening, but not before wetting the grounds and reducing FFG values across IA into N IL. Eventually, the wave and moderate rains will cross the Northeast by Friday with QPF generally around .5" at the eastern portions of New England to 1" further upstream in eastern Great Lakes...generally supported by a multi-model blend. Quick on the heels of the weak shortwave, the main upper level height falls will advance out of the Central Rockies late Thursday following a very similar track. The low level environment will have a short window to respond; however, it is likely lingering cloudiness will limit solar insolation for best instability to reach up to 1000-1250 J/Kg near the main surface low. Ample moisture with TPWs to 1.7" in a narrowing plume in advance of the frontal zone though flow will be less orthogonal to the frontal boundary also limiting extreme rates. Still, upper level support through DPVA and divergence along with the modest low level instability will allow for an additional line of convective development late Thursday into Friday across IA into NE KS...though there is some uncertainty given prior convection/outflow boundaries from prior wave. Currently, best dynamics and moisture intersection appear to set up a similar condition as the prior evening(s), when grounds will have been wetted in preparation for this round. As such a Slight Risk of Excessive Rainfall was introduced from eastern IA through Chicagoland area with a marginal extending across Northern IND and southern L.P. of MI for Day 3 (12z Thurs to 12z Fri). Toward the end of the forecast period (after 12z Fri), the preferred WPC blend of the 12z GFS/ECMWF transitions to a 12z GFS/00z ECENS mean blend as the ECWMF appears to become rapidly amplified by a secondary/stronger northern stream shortwave digging out of Canada. With reduced low level moisture convergence (as the front parallels the deeper flow) and lack of instability, QPF values will steadily reduce in magnitude across the eastern Great Lakes into the Northeast. ...Southern Plains... At 00z Thursday, main height-falls across progressing through the Desert Southwest and northern Mexico, strengthening the LLJ up the Rio Grande with rapid influx of moisture/retrograde of the dry line across the Pecos River intersection. Strong instability, but equally strong capping will be in place early but as the inversion starts to over turn or weak cell moving off higher terrain of the northern Sierra Madre will spark deeper convective initiation. Hi-Res CAMs are aggressive with the northern cells across Northwest TX but given their inconsistency with new convective development, would favor the more reliable hi-res versions of the Global GFS/ECMWF models were there is fairly good agreement with multiple recent cycles suggesting development to be closer to the Rio Grande with some additionally cells as far north as southern OK, along the dry line. The ECWMF is a bit further east with the dry line, leading to further uncertainty. The complex will shift north and east with time over Central Texas toward the DFW Metro throughout the day, likely to maintain a bit better/longer if the source development is closer to the Rio Grande which is better timed with the upper level support exiting Northern Mexico. A broad Marginal Risk is placed across the northern bend of the Rio Grande to Northwest TX to best account for the uncertainty when cells have the highest rain rates. A Slight Risk may be needed as the uncertainty reduces with additional model cycles. As the main wave lifts across the Central Plains to the Midwest late Thursday into Friday, the base of the trough becomes increasingly positive tilt, leaving a piece of the wave to linger in Sonora state, keeping moisture flux/return up the Rio Grande through the bulk of the period, while also leaving the lingering draped boundary toward the Big Bend Region. This is an ideal setup for large MCS to develop in the Day 3 time period, perhaps even toward the end of the Day 3 excessive period (06-12z Fri). WPC preferred a mix of the 12z GFS/ECMWF blend with respect to this wave/MCS...though traditionally, the guidance is too far northwest with convective initiation eventually shifting southeast down the River toward Del Rio as it approaches reality; at this point it is too difficult to break from the fairly tight agreement. There was a temptation to add a Marginal Risk to this area to account for potential for flooding associated with the MCS, but given dry conditions and lateness of development in the Day 3 excessive period will hold off at this time. Chenard/Gallina Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml