A Few Storms This Weekend, Uncertain Forecast Next Week

SE.fronts.20150829.07[1]  US850dew[1]

Moisture has been increasing across the area, as expected, as a low-level southeasterly flow has set up between the exiting high pressure to our northeast and low pressure in the Gulf of Mexico.  You can see, in the first image, that mid to upper 60 dewpoints have returned to much of the Deep South and Tennessee Valley.  This resulted in a few showers and thunderstorms over central Alabama on Friday, but most of the Shoals stayed dry, with the deeper moisture still remaining off to the southeast (as shown in the darker green colors on the graphic to the right above).  Moisture will continue to increase on Saturday as a deep-layer southerly flow sets up in response to the low pressure in the Gulf of Mexico (below, left image), but we still expect the deeper moisture to stay off to the south and east.  Because of this, we will continue to forecast a slight chance of a few isolated showers and thunderstorms (similar to what is shown by the 4 km NAM model, below, right), but the better rain chances will be off to the east and south.

USvort.20150829.00[1]  nam4km_ref_seus_7[1]

We will increase rain chances a little for Sunday, as the low pressure lifts out of the Gulf and moves across Alabama and toward Georgia, but we still expect the more widespread rain to be to the east of our coverage area.  The low in the Gulf lifts out on the area by Monday, but we will hold onto a slight chance of isolated storms, with the lingering moisture across the area.  Tuesday and Tuesday night should be mostly dry, as high pressure ridging temporarily influences the area before deeper moisture begins to move back northward from the Gulf of Mexico.  However, we will hold on to a slight chance for an isolated thunderstorm, since it is mid to late summer, and there will be a moist air mass in place.

05L_tracks_latest[1]

Here’s where the forecast begins to get very tricky for the middle of next week and beyond…

Tropical Storm Erika has failed to really get much organization all week, and this has kept her on a more westerly track than what had been anticipated.  This has caused Erika to run across the tall mountains of Hispaniola, and it’s quite possible that if you are reading this by Saturday midday or afternoon, Erika may be down to a tropical depression or even just an open tropical wave.  This lack of organization and a more westerly course has dramatically increased the chances of whatever remnants left from Erika of tracking into the Gulf of Mexico.  There won’t really be a shot of redevelopment, with a new trough of low pressure dipping into the Deep South and northern Gulf by midweek, but that will help to pull the remnant moisture from whatever is left of Erika northward toward the Florida panhandle.

This is where it gets tricky…

gfs_mslp_pcpn_frzn_seus_29[1]  ecmwf_T850_seus_8[1]

There are two different scenarios in the models with what happens with the remains of Erika once they move northward into the Florida Panhandle by midweek, and this will have a big influence on our forecast locally for the latter part of next week and into next weekend.  The GFS, on the left above, slows down Erika’s remnants over southern Georgia late in the week as a high pressure ridge aloft builds in to the north and blocks Erika’s remnants from ejecting northeastward.  The GFS then slowly moves those remnants westward through Mississippi and Alabama into next weekend before another trough approaches from the Midwest.  This would greatly increase moisture across the area, and would mean much better rain chances for the second half of next week and into next weekend.  This is a very plausible scenario.

The other solution, as shown by the Euro above on the right, brings Erika’s remnants northward toward the Florida panhandle also (or just east of there, toward the Big Bend area), but then slowly ejects them northeastward toward Georgia and the Carolinas or just off the coast, with remnant energy remaining in the area there for the rest of the week.  This would keep the deeper moisture associated with Erika’s remnants off to our east, and this would result in much lower rain chances.  This is also a plausible scenario, with strong winds aloft associated with the second Gulf low that will steer Erika’s remains northward possibly continuing that northward push out of the area.  However, we also caution that it looked like this run of the Euro model initialized Erika’s center of circulation tonight too far northward, off the north coast of Hispaniola… when, in fact, she has been tracking further south of there.  This northward bias has been in the models with Erika all along, and it has been wrong… and it’s very possible that this could cause the Euro’s track with Erika to be too far north/east, compared to what may really happen.  This gives credibility toward the GFS solution of the remnant energy increasing our rain chances late next week.  We just can’t be sure until whatever remains of Erika moves away from the islands and starts approaching the Gulf of Mexico.  We will begin to get a better handle on this as we head through the weekend.  As of right now, the forecast for the second half of next week is low confidence…


Changing Weather Ahead, But Nothing Too Bad

USvort.20150827.12[1]  USvort.20150828.00[1]

As we’ve been talking about, the big trough in the eastern United States that has brought our wonderful weather this week would not stick around too much longer.  You can see, in the images above, the trough lifting away into southeastern Canada, with the remnant energy we’ve also been talking about for the weekend meandering from Alabama and Georgia, back into the central Gulf of Mexico.  So far, the forecast has been going exactly as planned.

nam4km_T850_seus_8[1]nam4km_Td2m_seus_7[1]

In the lower-levels of the atmosphere, the flow is going to start to come more out of the east and eventually southeast, in response to the high pressure over West Virginia, and the weak surface reflection of the Gulf of Mexico upper-level trough.  This is going to mean an increase in moisture, as you see with the dewpoints back into the mid to upper 60s.  However, notice that the richer and deeper moisture is still to our southeast on Friday, as our low-level flow… while increasing the moisture some… is still originating from portion of Virginia and North Carolina, around the back side of the high pressure to our northeast.  This means that, while we can’t 100% rule out a stray shower for your Friday with the increasing moisture, the chances will be very very low.  The greatest chance of rain on Friday will be off to our southeast, in the deeper moisture plume over east central Alabama and into Georgia and Florida.

namconus_z500_vort_seus_15[1]  namconus_z500_vort_seus_23[1]

The remnant upper-level energy over the Gulf that we have been talking about starts to wrap up on itself a little and move northward through the weekend.  This will allow for a more southerly deep-layer flow and an increase in moisture, before the disturbance lifts to our northeast later Sunday evening…

namconus_apcpn_seus_22[1]

This will allow for a slightly greater chance for a few isolated to scattered showers and storms for both Saturday and Sunday, but with the disturbance tracking near our area or just to the east, the better dynamics for more widespread rain will be off to our east, as the accumulated rainfall projection through Sunday afternoon (shown above) would suggest.

We will still be in a deeper moisture field through the early to mid part of next week, which lends itself to the potential for more isolated showers and storms… especially during the heat of the day each afternoon.  With little confidence in any significant triggering mechanisms, as of now, we don’t think the rainfall will be too widespread or heavy.

In all honesty, next week’s forecast depends heavily on what Tropical Storm Erika does from now through the weekend.  The system has struggled greatly to organize so far, and this has allowed her to track more south/west than models had been anticipating earlier in the week.  This also puts her closer to some of the mountainous islands in the Caribbean, such as Hispaniola.  Disorganized tropical systems have a very hard time surviving encounters with those taller mountain ranges, and that makes Erika’s future very uncertain.  If Erika is able to maintain some organization and get north of Hispanola over the next day or so, it is likely that she will move through the Bahamas and either approach the east coast of Florida, or recurve toward the Carolinas (or possibly even out to sea).  HOWEVER, if Erika’s circulation is greatly disrupted by the islands and this causes her to weaken, this MAY allow her remnants to track more west-northwestward toward the southern areas of Florida… which may increase the chances of remnant energy getting into the Gulf of Mexico.  Some model solutions are starting to pick up on that idea.  We will state up front that IF that situation to play out, it would be a case where REMNANT energy from Erika would be moving into the Gulf, NOT AN ORGANIZED TROPICAL CYCLONE.  However, such a situation may mean a more widespread threat of showers and thunderstorms for the second half of the week.  On the other hand, if Erika tracks off to the east (which is the forecast consensus, and the official forecast from the National Hurricane Center, as this is being typed early Friday morning), Erika may actually help to pull drier air into our area by midweek and DECREASE rain chances here for the second half of the next week.

It’s a very low confidence forecast once we get past Monday, but we will be fine tuning next week’s forecast as we head through the weekend and things become more clear.  However, regardless of whether Erika tracks off to the east, or even the off chance her remnant energy gets into the Gulf of Mexico, we are NOT expecting a major threat from a tropical storm or hurricane on the Central Gulf Coast next week.  If something unforeseen happens and that changes, we will have further updates right here on shoalsweather.net…


Warmer Ahead with a chance of Rain…

Another lovely day in the weather department, but things are currently changing as more typical late summer weather will be on its way back.  Surface high pressure is currently in the process of shifting East of the area.  Northeasterly flow at the surface will switch to the East and eventually Southeast which will usher in more humid air.

Surface map:

usa_ICast

Friday will be the last totally dry day, but it will feel a bit more humid as moisture ushers in from the East.  Highs tomorrow should make it into the upper 80s with a few cumulus clouds being able to pop with the added moisture.  The troughing that has dominated the area will begin to cut off and retrograde just a bit to the West for Saturday and Sunday.  This will help with more of a flow from the Southeast at the surface and also in the lower and middle levels of the atmosphere.  Isolated rain chances will come back for the weekend as enough moisture and atmospheric lift is over the region.  Highs for Saturday and Sunday will reach the upper 80s and lows will return to near 70.

A look at the 500mb pattern for Saturday shows the baggy upper level trough over the region providing enough of a trigger for isolated storms.

gfs_namer_060_500_vort_ht

The trough will begin to lift out as the cut off low will begin to retrograde toward Texas by Monday,  This will keep the area generally under weak Southeasterly flow.  Isolated storms will continue Monday through Thursday as moisture lingers and diurnal heating allows enough instability for storms to pop.  Right, now the deepest moisture appears to remains East of the region associated with Tropical Storm Erika.  Currently, models recurve Erika to the East of the area, it will likely pull down some drier air on the Western side.  That will eliminate the rain chances into later next week.  Highs for your Monday-Thursday will remain in the upper 80s with nightly lows near 70.