Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 417 AM EDT Wed May 16 2018 Prelim Day 1, Day 2 and Day 3 QPF Discussion Valid May 16/1200 UTC thru May 19/1200 UTC Reference AWIPS Graphics under...Precip Accum - 24hr Day 1... ...Mid-Atlantic/Central and Southern Appalachians... Southerly flow along the western periphery of the Bermuda high and ahead of a weakening trough lifting out of the eastern Gulf of Mexico will continue to channel deep moisture northward through the region. This moisture along with weak mid-level energy lifting out ahead of the trough is expected to support some orographically enhanced rainfall totals along the eastern slopes of the southern to central Appalachians. Guidance shows a fairly good signal for south to north oriented training convection producing some locally heavy amounts along the favored terrain from far northeast Georgia and western North Carolina, to at least as far north as southwest Virginia later today. As energy continues to lift north, weak surface to low level wave development will enhance moisture convergence, which along with some support from the right-entrance region of the upper jet, may generate another round of heavy rains this afternoon and evening from central and eastern Virginia to along a stationary boundary draped across the northern Mid-Atlantic. ...Southeast... Onshore flow and low level convergence east of the surface wave associated with the previously noted upper trough is expected to support some moderate to heavy amounts across the eastern Florida panhandle into southern Georgia today. Heavy rainfall threat is expected to wane by early Thu as the low weakens and lifts further north. ...Pacific Northwest/Northern California/Great Basin/Northern Rockies... A weak mid-upper level low is forecast to drift northeast from the northern Great Basin into the northern Rockies this today. Guidance shows PWS increasing to around an inch (over 2 standard deviations above normal) near the center. This moisture along with daytime heating is expected to support scattered storms with the potential for locally heavy amounts from central Oregon northeastward into western Montana. Meanwhile, an upstream low is forecast to move into northern California, fostering additional precipitation further south and west across northern California into northern Nevada late Wed into early Thu. Days 2/3... ...Pacific Northwest/Great Basin/Northern Rockies... A long wave trough trundling 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 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 00z ECMWF/GFS/NAM. Day 2... As the long wave trough moves slowly eastward 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 on the Northern Rockies. Model soundings over northern ID and nearby western MT showed MUCAPE values in excess of 500 J/KG in these areas between 17/18z and 18/03z. The flow associated with the long wave trough keeps precipitable water values between 0.75/1.00 inches (which is about two standard deviations above the mean), and the resulting convection is expected to produce an axis of 0.75/1.25 inches of QPF over far northern ID and the Rocky Mountain Front Range in MT. Three hour flash flood guidance values are as low as an inch, and the National Weather Model showed high stream flows, so a Marginal Risk was placed here for Day 2. Day 3... 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 synoptic scale ascent associated with the mid level system , as well as marginal instability, is expected to produce mainly diurnally driven convection across the higher terrain of the Intermountain West. 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. With this in mind, the WPC QPF was based on a blend of the 00z ECMWF/GFS/NAM. Day 2... A 30 to 40 knot low level southerly flow transports 1.25 inch precipitable water air 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 00z ECMWF/GFS showed local QPF amounts in excess of 2.00 inches along this axis, but in different locations. The storms are expected to be outflow dominated after 18/00z, but after that, local training could result in a flash flood risk. Thus, a Marginal Risk was placed here to account for the threat. 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. There is some model signal for a cluster of storms or an MCS to form on the front and track eastward. Depending on the placement of the front (as there is still some spread concerning this), some of the instability could be elevated. In any case, there is a multi model signal for 1.50/2.50 inches of QPF over southern ND into northwest MN. Three hour flash flood guidance values are as low as 1.00/1.50 inches along the ND/MN border, and the National Water Model showed high stream flows before 18/12z. Because of this, a Slight Risk was placed over much of southeast ND for Day 2. Day 3... 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. Both the 00z NAM/GFS showed local 4.00+ inch QPF amounts here. Training of cells would need to occur for these amounts to be realized, which is possible near the front. Based on the above, a Slight Risk was placed here for Day 3. ...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 00z EMCWF/UKMET seem to have the best handle on the frontal position, keeping it further south than the 00z NAM/GFS. Based on this, the WPC QPF was based on a multi model blend, weighed more heavily toward the 00z ECMWF/UKMET. Day 2... Ongoing convection across the Mid Atlantic early on Day 2 should weaken early as it exhausts any remaining instability. After that time, a frontal boundary extending from the OH Valley across the Mid Atlantic states will waver, as surface high pressure builds across Quebec, and tropical moisture (with precipitable water values near 1.75 inches, which is between two and three standard deviations above the mean) continues streaming north in association with a broad long wave trough extending from the OH Valley to the Gulf Coast. A south southeast inflow increasing to 20 to 30 knots along and ahead of the surface front, which lifts slowly north after 18/00z. There was a fairly strong model signal for 2.00/3.00 inches of QPF across eastern WV into northeast MD, though there are some important differences in the model placement. As mentioned earlier, a more southern solution is preferred with this front, as instability will be the key to how far north the highest QPF can get during the last part of Day 2. An axis of 2.00/2.50 inches was stretched across north central VA into central DE, with another axis of 1.50/2.00 inches extended down through the Southern Appalachians into western NC. Given the low flash flood guidance values over WV/nearby western VA, a Moderate Risk was considered for this area for Day 2. However, since there is still some important model spread on the placement of instability and higher QPF, this was eschewed this morning, in favor of a Slight Risk extending from west central NC across eastern WV, much of northern VA/MD into central and northern DE. A Marginal Risk was placed north of here to account for model spread. Day 3... While there is better model agreement with the placement of the front across the northern Mid Atlantic/Northeast on Day 3, much of the 00z guidance may still be too far north with the front. The surface high exits early in the day, which could allow the front to move northward, but instability could still be the key to where the best convection occurs. Some of the convection is expected to be elevated, especially across southeast PA across central NJ into the NYC Metro area, but the depth of the tropical moisture could allow elevated convection to produce heavy rainfall. An axis of 1.75 to 2.25 inches of QPF was placed along this axis. 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. Day 2... A broad long wave trough over the Gulf Coast and Southeast states continues to focus tropical moisture and moderate instability across the region during Day 2. Mid level forcing associated with the long wave trough is expected across the Mid Atlantic states, so much of the convection here is expected to be diurnally driven. For the most part, QPF amounts are expected to remain below 1.00 inch, except where local training can occur with slow cell motions. The exception here may be the east coast of FL, where the mid level flow parallels the coast. A 20 to 30 knot low level southerly flow continues to supply 1.75+ inch precipitable water to the coast, where model soundings showed moderate instability. Much of the convection is expected to be diurnally driven, with an area of 0.50/1.00 inches of QPF placed here. However, training could produce local amounts over 3.00 inches over some of the urban areas from Miami up through the Space Coast. Because of this, a Marginal Risk was placed here for Day 2. Day 3... Similar conditions are expected during Day 3 across the area, with the best low level flow expected once again over the east coast of FL. Since training is possible over the urban areas of the FL east coast, a Marginal Risk was placed here for Day 3. Pereira/Hayes Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml