WEDNESDAY: 12-AUG-15
(Next Regular WX Update FRIDAY- AUG 14)
*** SPECIAL UPDATE ON EL NINO TOMMOROW (THURSDAY, AUG 13) ***
TROPICAL STORM HILDA WEAKENS RAPIDLY – REMNANTS TO PASS SOUTH OF THE BIG ISLAND
After holding up a bit longer than originally expected - HILDA finally succumbed to the strong westerly shear overnight - and while it’s officially still being carried as a tropical Storm, the cyclone is almost certainly down to Tropical Depression intensity. The cyclone will continue weakening and will likely become a remnant Low by tomorrow as it drifts westward – passing well south of the Big Island early Friday. While the system may yet produce locally heavy showers on the Big Island - no major impacts are expected anywhere across the state.
The tropical Atlantic remains uneventful – with the only area of modest interest found over Africa. Even with the large reduction in the SAL flow from Africa westward into the eastern ATL, tropical waves moving westward from over Africa to the EATL are still relatively weak and most are entangled with the ITCZ which is resulting in a poor environment for cyclone development. The GFS is the ONLY global model that even ‘hints’ at some weak cyclone formations in the EATL later next week. However, the timing of these cyclone formation forecasts continuously changes from one run to the next – a clear indication that these system formations are unlikely to happen. Only seasonality argues for a cyclone formation as we approach the seasonal ‘peak’ in storm formations that occurs in about 4 weeks.
WARM WEATHER WITH SURGES OF HEAT AHEAD
Even with the high amplitude (but relatively weak) upper air TROF over the eastern US, the sub-tropical high is generally the dominant, large scale weather system across the rest of the country and will become more influential in the eastern US during the next few days as the eastern US TROF weakens and lifts out to the northeast.
While the GFS continues to ‘flip-flop’ on how strong the sub-tropical High will be over the Midwest to New England – most of the major cycle model runs this past week (i.e. the 00Z and 12Z model cycles) are forecasting a major warm-up from the Midwest to Northeast starting this weekend and it will generally persist with only minor and short-lived cool downs during the next 2 weeks. Short wave TROFs moving from the Pacific and NW Canada will continue to be the “fly in the ointment” for just how warm or persistent the heat surges will be in the Midwest and Northeast. Because of the relatively good run-to-run model consistency, I’ve opted for the warm solution during Week 2. However, considering the poor track record of the GFS on these warm-ups during the Week 2 forecast periods - and seasonal forcing that becomes a significant factor during the 2nd half of AUG - overall confidence in the Week 2 Temp anomalies is very low in the Midwest region. Adding to the uncertainty is the potential for the development of a ‘ring of fire’ from the Plains to upper Midwest and Ohio Valley during Week 2 with periodic T-Storm development around the northern periphery of the sub-tropical high. This kind of development would lower average Temps significantly during Week 2.
ALASKA – FALL IS IN THE AIR
The seasonal transition to Fall will make its first real appearance over Alaska this coming weekend as a far more dynamic pattern emerges in the higher latitudes - with the first in a series of stronger storm systems impacting the western portion of the state, spreading to much of the southern half of the state later next week.The increase in storm activity will lower Temps to near or below normal over the state, with significantly cooler than normal readings expected over much of the state during the last 10-days of the month. The only real, widespread exception will be north of the Brooks range and North Slope where Temps will likely remain solidly above normal during the next 2 weeks due to a predominantly southerly wind flow. The only big question for Week 2 is just how stormy it will be as the most recent GFS model run now shows a Major storm formation far to the south of the state versus over the Aleutians and GOA as had been consistently forecast by the global models during the past week and is the 'typical' location for storm systems.
EL NINO / MJO
SST anomalies associated with El Niño have changed little over the past few days, with the latest reading for the Niño 3.4 region at +1.85˚C.
The MJO has strengthened slightly over the past 3 days, with the signal located over the far western Pacific. However, the apparent strengthening may be due to constructive interference from other low frequency modes of circulation such as atmospheric Kelvin waves and the well-entrenched El Niño. This is reflected in the forecast from all global models for the signal to actually weaken and become incoherent during the next 10 days. Either way, the MJO will not be a significant factor in the weather across the tropics through next week.
** SPECIAL EXPANDED UPDATE ON EL NINO will be posted tomorrow. (THURSDAY, AUG 13) **
**
Fig 1: IR (infrared) image of Tropical Storm HILDA with Shear Analysis from earlier this AM depicted a ‘storm’ being torn apart by the strong wind shear of over 40Kts across the storm. Convection has weakened tremendously, and the system is likely down to Depression intensity.
Fig 2: Early cycle model runs of the specialized Hurricane Forecast models for the Track/Intensity for HILDA. As mentioned in yesterday’s Special Update on HILDA, the track forecasts which were ‘all over the place’ due to differences in the forecasts for the rate of weakening, are now much more tightly clustered as the forecast for the rate of weakening has been settled. With the system now much weaker, the shallow remains of HILDA will be controlled by the easterly Trade Winds and the consensus is HILDA will track well south of the Islands during the next couple days before completely dissipating.
CLICK IMAGE to open full size image in a new windowFig 3: Color Enhanced IR (infrared) Overview image of the Tropical Atlantic There are only 3 tropical waves across the entire basin, and none of them pose any risk for development. Both of the TW’s in the CARIB and central ATL are interacting with upper level TROFs which is providing some lift and upper level divergence triggering some isolated convection, but even those are very limited.
Fig 4: ‘Enhanced’ VIS imagery over the eastern ATL and Africa. This experimental product highlights areas of Dust associated with the SAL, with the only area of dense dust confined to the immediate NW African coastal area just ahead of a TW now along the coast. This weak wave is followed by a slightly stronger one about 250NM to the east, and a better organized one further to the east. It appears that the eastern most TW has been picked up by the GFS model which develops it into a weak cyclone next week. (But as mentioned above, the GFS is almost certainly over-forecasting this development.) The fact that these waves are still embedded within the ITCZ makes it especially difficult for any of the disturbances to develop into a cyclone.
Fig 5: Computer generated forecast for Dust Transport across the Atlantic associated with the SAL. While the SAL (Saharan Air Layer) continues to be a factor in the potential development of a Tropical Storm – the recent reduction in Dust flow appears poised to increase again during the next few days with a significant surge expected to reach down to the Cape Verdes by the weekend. Still, the areal extent of the SAL remains far less than just a couple weeks ago.
Fig 6: Graphical representation of the MJO location and relative strength. The MJO signal has strengthened somewhat over the last few days (Left) but forecasts call for the MJO to weaken and/or become incoherent over the next week. Because the MJO is relatively weak – and should stay weak – it is not expected to have a significant influence on tropical weather (or El Niño) through next week.
*** SPECIAL UPDATE ON EL NINO Scheduled for this THURSDAY, AUG 13 ***
Fig 7: GFS Jetstream Forecast for the next 2 Weeks. At the beginning of the forecast period, we find an amazingly strong E-W orientated subtropical jet stream over the EPAC just to the north of the Hawaiian Islands, with a TROF in the eastern US and the sub-tropical high centered across the southern Rockies. Short wave TROFs off the PAC NW coast and just north of Alaska/NW Canada will move generally eastward - but are not expected to ‘dig’ southeastward in the eastern US. This will (should) keep cooler air bottled up in Canada thru most of the next 2 weeks. By the end of the forecast period, a large and unseasonably deep upper level Low is expected to develop over the EPAC to the south of the GOA (Gulf of Alaska) with the sub-tropical high still dominating much of the lower 48 - while a large, upper level Low and TROF begins forming over northern Canada. (I hate seeing this too...) In general, you can see the gradual strengthening of jet stream wind speeds across all of the higher latitudes as we head into the fall.
Fig 8: Temperature ANOMALY forecast for Week 1 is based STRICTLY on the MOS data forecasts from the todays 12Z operational GFS model run with minor adjustments towards the raw model data points. Above normal readings across much of the nation with most of the increase in the Midwest and Northeast occurring starting this weekend. (The somewhat lower anomalies in the inter-mountain west is due to localized Precip associated with the Monsoonal flow.) Confidence in the forecast anomaly Pattern and magnitude is well above average with readings of ‘5’ and ‘4’ on a Scale from 1 to 5 for the anomaly pattern and magnitude respectively.
Fig 9: Running Precipitation totals for US during the next 10-days (and only for totals > 0.75”) based STRICTLY on the GFS Heaviest Precip will continue in the far Southeastern US, with a secondary region from the Great Lakes to central Plains – along with an increase in shower activity along the Gulf coast from Florida westward to the TX coast
Fig 10: The Week 2 Temperature ANOMALY forecast is based on the 12Z run of the GFS (90%) integrated with the 12Z GFS Ensemble (10%) using the projected pattern along with explicit surface and 850mb (~5,000 FT) Temp forecasts. Some Temps are adjusted for known / expected anomalous thermal patterns & projected storm system passages.A VERY warm – even Hot - period is likely across the nation – especially in the Midwest and Ohio Valley. However, there is VERY HIGH uncertainty for the forecast in this region, with some of the uncertainty due to the tendency for cold fronts to move further south than GFS forecasts call for in the extended ranges and the potential for significantly more T-Storm activity from the Plains to Ohio Valley eastward to the Mid-Atlantic states. Overall Confidence in the forecast anomaly Pattern and magnitude is now quite low, with readings of ‘2’ for the anomaly pattern, and only ‘1’ for the magnitude on a scale of 1 to 5. (However, most of the uncertainty is in the Midwest and Northeast.)
Fig 11: Today’s Temperature anomaly forecasts are based on the 12Z run of the Operational GFS using the GFS MOS data Stormier weather will begin lowering Temps over the state during the week ahead – with below normal readings statewide likely during Week 2.
✭ The Next REGULAR Weather Update will be on FRIDAY AUGUST 14 ✭
SPECIAL UPDATE ON EL NINO TOMMOROW (THURSDAY, AUG 13)
Want to be alerted when I post an Update? Then follow me on Twitter.
Steve
NOTES:
1. A GENERAL Glossary of Weather Terms can be found HERE
2. Another Glossary of weather terms is available HERE
No comments:
Post a Comment