Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1157 AM EDT Wed May 22 2019 Valid 12Z Sat May 25 2019 - 12Z Wed May 29 2019 ...Record/Dangerous Heat for the Southeast this weekend into next week... ...Heavy rainfall possible for portions of the central Plains/Corn Belt... ...1600 UTC Update... Guidance is still finding it a challenge to resolve separate pieces of energy that contribute to the overall closed low dropping southward near the West Coast and then ejecting inland--a leading compact upper low forecast to reach Oregon by early Sat and another feature about 18-24 hours behind it. Current guidance differences and changes in continuity (especially for the trailing energy) by themselves temper confidence in exact details for much of the period. In addition by days 5-7 Mon-Wed there are still issues over how much energy may push into the eastern Pacific ridge aloft as well as amplitude/depth of cyclonic flow over southern Canada. Generally the GFS/CMC cluster that brings more energy into the eastern Pacific leads to a sharper western Canada ridge and in turn more southern Canada into northern U.S. troughing. This ultimately results in faster ejection of the western U.S. system and then more phasing to the east. On the other hand ECMWF mean runs thus far suggest the 00Z ECMWF could become too slow with bringing at least some height falls into the northern Plains by next Wed. Over the past day the GEFS mean has trended toward holding onto somewhat more eastern Pacific ridging, which combined with typical biases that favor slower breakdown of upper ridges, favor maintaining a forecast that tilts more toward the ECMWF/ECMWF mean in principle later in the period. However by next Wed latest operational ECMWF runs offer some question marks. The latest 00Z run may be a little slow to bring at least some height falls into the northern Plains while the prior 12Z version had no southern Canada troughing in contrast to latest consensus including the ECMWF mean. The updated days 3-5 operational model blend uses the 00Z GFS/06Z FV3 GFS for the GFS component due to those runs being slower than the 06Z operational GFS for the ejecting western system, while employing varying weights of the 00Z ECMWF/UKMET/CMC. By the end of the period the forecast places greatest emphasis on the 00Z ECMWF mean and continuity. GEFS mean trends over the forecast domain have brought it close enough to preference to allow for some inclusion of the 06Z run. Rausch Previous discussion issued at 0618 UTC... ...Overview... Rather stagnant but highly amplified upper level pattern will continue to be dominated by deep troughing in the west and strong ridging in the Southeast. A closed low will sink through/along coastal California this weekend to bring another round of precipitation to the west under a chilly airmass. The Northeast/Mid-Atlantic will lie in the confluence between the northern and southern stream as a wavy boundary oscillates initially over the Carolinas lifts northward as a warm front before a cold front pushes out of the Midwest into the Mid-Atlantic late Sunday. The pattern favors hot/dry conditions in the Southeast with record temperatures near and just over 100F. In between, broad southwesterly flow aloft and southerly flow at the surface will favor rounds of convection and locally heavy rainfall with multi-inch totals over the period especially over the central Plains into the Corn Belt. ...Guidance Evaluation and Preferences... Latest guidance proved to be a good consensus/starting point for the forecast, though the 12Z UKMET was quicker than the GFS/ECMWF/Canadian with the upper low in the west Sat. In general, a blend of the GFS/ECMWF added some detail to the forecast through about next Mon/D5 given some continued differences in timing/track of the northern stream (especially across Canada due to difficulties resolving anticyclonic wave breaking east of Alaska). Next week, ECMWF and its ensembles were slower to eject the upper low out of the west compared to the GFS/GEFS mean while the Canadian was in between. Favored only the ECMWF/ECMWF ensemble mean by next Tue/Wed as the trend has been for a slower speed and since the GFS/GEFS typically is too quick by ~6-7 days out with initially amplified features. However, trough may eject in pieces rather than wholesale per some ensemble members. Ensembles favor a more positively-tilted trough by next Wed with a flatter ridge in the Southeast and western Canada. ...Weather Highlights and Hazards... Sustained heat will become increasingly intense in the Southeast into next week with widespread temperatures in the upper 90s and a modest area of 100-103F which will challenge monthly records. Overnight lows may slowly struggle to fall to 70F which will pose additional health stresses due to the early timing of this heat, more typical of June/July. Heat indices over 100F are likely for many areas under the upper high with no chance of rain due to the stable atmosphere. Temperatures will trend toward average farther north along the east coast. The west, by contrast, will see well-below average temperatures by about 15-25F with abundant cloud cover and modest rain with some high elevation snow. Some light rain is possible over most of California (though focused over the Sierra) even down to southern areas but probably not into the deserts. To the east, temperatures around the wavy boundary from the central Plains eastward will rise/fall as the front lifts northward or slips southward. This will also provide a focus for rainfall and some convection. Rounds of rainfall with locally heavy amounts may add up to several inches over the period, some of which may fall within a short amount of time over areas that have seen quite a bit of rainfall recently. Greatest flooding/flash flooding risk during the period seems to be centered over central/eastern Kansas northeastward through Iowa into Wisconsin by next Wed. Fracasso Additional 3-7 Day Hazards information can be found on the WPC medium range hazards chart at: https://www.wpc.ncep.noaa.gov/threats/threats.php WPC medium range 500mb heights, surface systems, weather grids, quantitative precipitation, winter weather outlook probabilities and heat indices 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/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml