Extended Forecast Discussion NWS Weather Prediction Center College Park MD 159 PM EST Mon Jan 01 2024 Valid 12Z Thu Jan 04 2024 - 12Z Mon Jan 08 2024 ...Heavy precipitation likely to impact areas from the Gulf Coast northeastward Friday into next weekend... ...Overview... The medium range period features a rather progressive flow pattern as a parade of storm systems traverses the country from West to East. One will be exiting the East Coast as another moves through the Southwest on Thursday, with potential for another round of heavy rain across the Lower Mississippi Valley/Southeast by Friday. As this system lifts up the East Coast next weekend, it has potential to bring heavy precipitation (rain to the south, snow to the north) to much of the East. Out West, the next system looks to move in next weekend as well with another round of valley rain and mountain snows progressing from California into the Four Corners region. ...Guidance/Predictability Assessment... Guidance agrees on the existence of the two dominant systems of interest, the system progressing from the Four Corners through the East Thursday into the weekend and then the amplifying upper trough over the West, but with some important detail differences especially for the leading system. There is somewhat more spread for a potential smaller scale system approaching the Pacific Northwest around Friday. For the system initially emerging from the Four Corners on Thursday, the most pronounced difference among the 00Z/06Z guidance was that the GFS was somewhat slower than other solutions plus tracked the primary low into the Ohio Valley while developing a secondary coastal/western Atlantic low--versus the others that maintained a single surface low from the Gulf Coast northeastward through the Southeast Coast and beyond. While the 12Z/31 ECMWF shared the general 00Z/06Z GFS idea, 00Z trends were pronounced away from the GFS, with the GEFS mean nudging more toward the ECMWF/CMC/UKMET cluster. The 00Z ECMWF-initialized machine learning models strongly favored the ECMWF/CMC/UKMET cluster as well. In the new 12Z model runs, the GFS has come in much closer to the majority while a faster trend in the UKMET still leaves quite a bit of timing spread. There are issues with exact northward extent of the track as well. Medium to smaller scale shortwave details with lower predictability may have a significant effect on the surface evolution, so additional adjustments in the guidance consensus appear likely. Well behind this system, there is a general clustering that favors a progressive shortwave and compact developing surface low nearing the Pacific Northwest around Friday. 00Z/06Z GFS runs were on the deep side with the surface low while the new 12Z run has tempered its depth a little. The 12Z GEFS mean hints at this system and the UKMET is farther north, while CMC runs do not reflect this system. Other guidance suggests the 00Z ECMWF becomes too amplified with the associated upper trough once it reaches the Plains by late Saturday. The Sunday-Monday upper trough amplifying into the West currently seems to have better than average predictability given its large scale and favorable guidance clustering which suggests an embedded upper low (whether actual, or implied as in the ensemble means) tracking near the Four Corners as of early Monday. A model/mean consensus looks good for this system. The above guidance comparisons led to updating the forecast with a 00Z/06Z model blend early and then incorporating some 06Z GEFS/00Z ECens mean guidance with the models later on, though a fair amount of operational model input was able to be maintained into day 7 in light of ensemble mean support for the amplifying western upper trough/associated surface system. Due to considerations for the system affecting the East, GFS weight was kept a little lower than would typically be the case. ...Weather/Hazards Highlights... Precipitation across the Four Corners/southern-central Plains starting by Thursday should become more widespread as the first of a series of upper lows taps Gulf of Mexico moisture to fuel another round of moderate to locally heavy rainfall across the Lower Mississippi Valley/central Gulf Coast states and perhaps nearby areas on Friday. The Day 5 Excessive Rainfall Outlook maintains a Marginal Risk area across this region, with latest guidance supporting some eastward expansion of the risk area. There is some question about how far northward instability may extend, but anomalous moisture and a leading front forming near the Gulf Coast to provide a focus, plus fairly wet antecedent conditions from short-term rainfall, seem to suggest some flash flooding potential. Heavy precipitation will spread across the Southeast/Mid-Atlantic and possibly the Northeast next weekend, with increasing potential for snow to the north and west of the low. Exact amounts and precipitation type for some of the major East Coast cities remains highly uncertain at this point, but does need to be watched. The WPC winter weather outlook probabilities highlight the best potential for at least 0.25" liquid in the form of snow over the central Appalachians. Just to the east, guidance trends have led to somewhat higher probabilities versus the previous 12-hourly issuance (for example, from 10 percent to 30 percent) near the DC-NYC corridor. It may take some time to resolve important details given the current spread for surface low timing/track. A smaller-scale system may bring a brief focus of enhanced rain/mountain snow to the Pacific Northwest around Friday. Then expect broader coverage of moderate to locally heavy precipitation from the West Coast/California and into the interior West by next Monday. There is a decent signal for meaningful mountain snows across parts of the Cascades, Sierra Nevada, and interior mountain ranges/southern Rockies. Surface cyclogenesis across the south-central Plains early next week should allow precipitation to move into the central U.S., with potential for snow to the north of the low track. Above normal temperatures should be mostly confined to the far northern tier with anomalies generally plus 5-15F for highs, and perhaps as high as plus 15-25F for morning lows over parts of Minnesota/Wisconsin during the weekend. Some moderately above normal temperatures may return to the Southern Plains/Lower Mississippi Valley early next week. The southern two-thirds of the West should see below normal temperatures late this week, with some cooler anomalies at least 10F below normal for highs spreading across the West into the High Plains by Sunday-Monday as upper troughing moves over the region. Northern parts of the West should trend below normal as well. Most of the East will likely see near to below normal temperatures late week into the weekend, perhaps rebounding toward normal by next Monday. Rausch/Santorelli Additional 3-7 Day Hazard information can be found on the WPC medium range hazards outlook chart at: https://www.wpc.ncep.noaa.gov/threats/threats.php WPC medium range 500mb heights, surface systems, weather grids, quantitative precipitation forecast, excessive rainfall outlook, winter weather outlook probabilities, heat indices and Key Messages are at: https://www.wpc.ncep.noaa.gov/medr/5dayfcst500_wbg.gif https://www.wpc.ncep.noaa.gov/medr/5dayfcst_wbg_conus.gif https://www.wpc.ncep.noaa.gov/5km_grids/5km_gridsbody.html https://www.wpc.ncep.noaa.gov/qpf/day4-7.shtml https://www.wpc.ncep.noaa.gov/#page=ero https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml https://www.wpc.ncep.noaa.gov/#page=ovw