IDA Nearly Gone / Mild Temps Thru Next Week / Locally Heavy Rain in the East

By: Steve Gregory , 8:10PM,GMT on September 25,2015

IDA Nearly Gone / Mild Temps Thru Next Week / Locally Heavy Rain in the East

FRIDAY, 25-SEP-15 / 3:10 PM CDT
Next Update will be on Monday, SEP 28

ONLY 'SHORT UPDATES THRU LATE OCTOBER !

I am truly sorry for the shorter updates – major hardware problems and an unexpected personal emergency is forcing me to drastically cut back on the time devoted to these updates. With a little luck, these shortened versions will be over in about 2 weeks – but quite possibly could take 4 weeks. Hopefully, you will ‘stick with me’ as I work through these issues.

Steve
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MILD PATTERN WITH TROPICAL RAINFALL IN THE SE US

With broad high pressure across much of the nation, well above normal Temps will continue thru the end of next week. At the same time, deep, tropical moisture flowing northward from the CARIB into the GOM is interacting with an old frontal boundary near the Carolinas, and will bring heavy rains to the SE US northward into New England during the next few days.

Short wave TROFs moving from the NORPAC into the PAC NW every few days will tend to amplify as they move across the Midwest, and on into the Northeast - with an especially strong amplification expected to develop in 8-12 days. This stronger TROF development is likely to bring the first significant cool air surge into the eastern half of the nation during Week 2. At the same time, a building ridge in the western US will send Temps to well above normal levels – with the possibility of a strong – but seasonal - Santa Ana wind event in southern California.

Longer term, with a very progressive pattern still expected to continue across the northern hemisphere as we head into mid-October, the deeper TROF over the eastern US during Week 2 is likely to lift out and de-amplify by mid-month, allowing Temps to rebound to near or above normal levels in the eastern US.

TROPICAL ATLANTIC

With the exception of Tropical Depression IDA in the central Atlantic (which is expected to dissipate over the weekend) – there are no significant tropical Waves across the tropical Atlantic basin. However, as mentioned last Monday, the western CARIB appears primed to develop a tropical Low or possible cyclone during the next few days. The latest GFS does not show a significant cyclone formation, but deep layered, tropical moisture is expected to flow northward towards the eastern GOM and into the SE US – increasing the likelihood of heavy or downright torrential rains, into the SE US.



CLICK IMAGE To Open FULL Size image in a new window
Fig 1: Overview of Tropical Atlantic BasinTropical Depression IDA is hanging in there over the central Atlantic, but should dissipate during the next few days. A Tropical Wave is approaching the west African Coast, but is relatively weak and is unlikely to develop as it moves slowly W/NW. Elsewhere in the tropics, deep moisture is found across the western CARIB and is flowing northward, with some indications of a broad area of Low Pressure forming over the NW CARIB. This system, and/or the tropical moisture associated with it, should move slowly northward into the eastern GOM during the weekend, with a slight chance of a weak cyclone forming in the GOM early next week.


Fig 2: Visible Image of Tropical Depression IDA The VIS image of IDA shows the low level circulation center well to the west of what little convection is associated with the system to the east of the center. The system has probably already weakened to below depression intensity – but will certainly do so during the weekend.


Fig 3: 500mb GFS FORECAST LOOP A generally zonal and progressive flow pattern should continue across North America during the next 7-10 days before a deeper TROF begins to form over the eastern half of the nation during Week 2.


Fig 4: Temperature ANOMALY forecast for Week 1 is based STRICTLY on the MOS data forecasts from the todays 12Z operational GFS model run with only minor adjustments towards the raw model data points for Days 6 and 7. Unseasonably warm Temps will continue nationwide thru next week as broad high pressure aloft remains in place into early next week, before starting to weaken later in the week. Confidence in the forecast anomaly Pattern and Magnitude is above average for this time of year, with readings of ‘4’ for both the anomaly Pattern and Magnitude on a Scale from 1 to 5.


Fig 5: GFS Precipitation forecast for the next 10 days (Cool Season 0.25” or more) Heavy Precip is expected over much of the eastern US during the next 10 days, possibly reaching into New England if a tropical disturbance forms and moves up the coast. Precip in the Midwest is associated with 2 short waves TROFs and associated cold fronts expected to move across the region around the middle of next week, and again towards the end of the 10 day period.


Fig 6: The Week 2 Temperature ANOMALY forecast is based on the 12Z run of the GFS (80%) integrated with the 12Z GFS Ensemble (10%) and the 00Z EURO model 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. Much cooler Temps will develop across the northern Plains, Midwest and Ohio Valley as an upper level TROF is expected to develop and amplify over the central/eastern US during Week 2. Overall Confidence in the forecast anomaly Pattern is near average now – but confidence in the anomaly Magnitude remains below average for Week 2, with a reading of ‘2’ on a Scale of 1 to 5.

✭ The Next WX Update is Scheduled for MONDAY, SEP 28th ✭

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Steve