Warm September in Much of US So Far; Three Cool Troughs Through the Third Week of the Month

1. Three cooling troughs come into the Central and Eastern states during the next two weeks to knock current warm departures back.

2. Pattern flip is possible during the last week of the month ending up with a cool trough in the West and a warming westerly flow and ridge in the Plains and much of the East.

3. One last cool trough for the Northwest states early this week then ridging favored in the West through the third week of the month will promote a period of above-normal temperatures.

4. There may be an upper-level low off the California coast later in week 1 through week 2 drawing some moisture and cooler air into the Southwest and California, while relative to normals, it is warming in the Northwest at the same time.

5. Upslope flow into the northern/central Rockies could bring mainly below-normal temperatures late this week through next week with chances for precipitation in the central/southern Rockies.

6 Plains and Midwest will get two more cool troughs with only brief visits of warmth through week two then a general warm spell the last week of the month lasting into early October.

7. In the eastern half of the U.S., positions of troughs during the next two weeks will lead to below-normal departures mostly west of the Appalachains and into the Southeast while areas from the Middle Atlantic states to New England east of the mountains stay around normal to slightly above normal on average.

The month of September has been warmer than normal across the bulk of the U.S. with strong westerlies across the northern U.S. thanks in part to a positive AO and positive NAO (progressive flow). The jet across the north has favored a broad ridge to its south. Here is a look at the mean 500 mb flow pattern since the start of the month (on the left) and the resultant temperature anomalies (on the right):

All of the lower 48 states with the exception of Florida, the northern Rockies, California and southwest Arizona had temperatures a few to several degrees above normal since the beginning of the month. The "core" of the greatest positive temperatures has been over the southern Plains (6 C or 11 F above normal) with a second "warm core" of 4 C or 7 F above normal over the Northeast. The next couple of weeks look quite different. This zonal jet has since buckled/amplified since late last week. The main reason for this is some temporary Atlantic blocking and the NAO sliding down towards neutral or just slightly negative. Here is a look at our Sunday evening flow pattern at 500 mb:

Now the temperature departures for Sunday:

Remember back to a week or two ago when we had the hint that this was going to happen? Around the last couple of days of August, we saw two typhoons in the far-western Pacific recurve, one of which was Typhoon Tembin, the second storm in a short time that ravaged Korea with floods. Looking at the flow around the hemisphere, one can see that amplification or blocking is present slowing the jet in some places. This is also tapping some of the colder air that was previously trapped in the Arctic regions when the AO was running strongly positive (close to +2). Granted the AO hasn't dropped all the way to negative, but it has decreased to under +1 anyway. Looking ahead, the blocking in the Atlantic is not very strong, so that monstrous trough you can see is also ready dropping southeast from Gulf of Alaska into western North America. In fact, look what the models are doing already by the middle of the week. Here comes trough number two. Here is a comparison of the GFS and the European ensemble means Wednesday evening, Sept. 12 (day 3):

The warm air in the West presently will be quickly forced east into the Plains this Tuesday and Wednesday as the Gulf of Alaska system plows through the Northwest into the northern Plains led by another strong cold front which will trigger more strong to severe thunderstorms Wednesday/Thursday followed quickly by another cooldown. The first Eastern trough gets kicked out with ridging taking its place. Days will warm back up in the East, but nights will stay cooler than normal under the dry surface high meaning overall departures only recover slowly. The Northwest will plunge to several degrees below normal through Tuesday then recover back to well above normal by next weekend as the trough departs. The next cool, Canadian high coming south into the Plains at the end of the week, the one behind the cold front with trough number two will bring temperatures back below normal southward through the Rockies and across the northern half of the Plains on into the Midwest. Apparently the trough in the Plains late this week will not be the last one. Here are the ensembles next Tuesday evening, Sept. 18 (day 9):

Let's hope that the European is overdone with the "monstrous" chilly trough in the Plains/Mississippi Valley. Now, the GFS ensemble means are not quite as amplified, but they are similar with the development of a polar vortex over northern Hudson Bay which would act to bring colder air down from a colder source region. Another similarity is the western North American ridge which is in such a position to produce a major East Coast storm system. In any event, I would say we are looking at below-normal temperatures at the end of the 6- to 10-day into the 11- to 15-day period from the Rockies to the Appalachians. The West will be "strange" in temperature distribution. The trough off the California coast may bring below-normal temperatures over California and the Southwest, but the ridge farther north could bring above-normal temperatures over the Northwest. Finally, here is the modeling at the end of the 15-day period, Monday, Sept. 24 (day 15):

It's interesting that both ensemble means show the pattern "flattening," probably because the individual members are in less agreement, but the last few runs of the GFS operation are now trying to put a trough in the western/central U.S. We have reason to believe that a trough may actually settle over the West during the last week of the month. Here are the European weeklies for week 3, Sept. 24 through Sept 30, issued this past Thursday. The mean 500 mb flow pattern is on the left with the temperature anomalies on the right:

The European is definitely hinting at trough over the Pacific states and notice the above-normal temperatures forecast to return to the Plains/Midwest/Great Lakes during the last week of September. Really, only minor adjustments were needed to our September forecast at this point and here is a look at that forecast once more:

I think any changes that would have to be made by next week if current trends continue would possibly be some lowering temperatures over the Southeast, the southern half of the Plains to the Ohio and Tennessee valleys, and some raising of temperatures in the Pacific states. One other thing that supports the trough back in the west late in the period would be the majority of the GFS ensemble members PNA forecast to go negative towards the end of the month:

Looking at analogs, with the trends I have been seeing since mid-August, I think 1986, 2002 and 2009 have become less of a match and two other ENSO years, 1957 and 1979 are looking better. Combining the Augusts of 1951x2, 1957, 1979 and bringing back 2001x2 and 2006, the average mean 500 mb flow and temperature departures for this new analog package looks like this:

Compare these to August 2012:

August 2012 ended up a little more amplified flow-wise, but the temperature distribution is a decent match though August 2012 by itself is warmer in the West and the Northeast. So what does this analog package yield for September?

I am still not happy with the analog years that we come up with, but hopefully we can latch onto something in the next couple of weeks before getting into autumn. The package certainly looks too cold in from the Rockies to the Appalachains for sure, but remember how warm this month has started this year. If you figure that in, then the analogs may just be showing what we are forecasting, the three troughs into the Central/Eastern states during the next two weeks. I think the warmth west of the Rockies is coming as well but maybe not over the Southwest as indicated here.

Have a great week!

Report a Typo