WPAC: NEPARTAK - Post-Tropical

[Hayabusa]

90W.INVEST

90W.INVEST.15kts.1010mb.22N.142E

[Hayabusa]

[Hayabusa]

ABPW10 PGTW 220600
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/SIGNIFICANT TROPICAL WEATHER ADVISORY FOR THE WESTERN AND
/SOUTH PACIFIC OCEANS/220600Z-230600ZJUL2021//
REF/A/MSG/JOINT TYPHOON WRNCEN PEARL HARBOR HI/220152ZJUL2021//
REF/B/MSG/JOINT TYPHOON WRNCEN PEARL HARBOR HI/220151ZJUL2021//
NARR/REFS A AND B ARE TROPICAL CYCLONE WARNINGS.//
RMKS/
C. SUBTROPICAL SYSTEM SUMMARY:
(1) AN AREA OF CONVECTION (INVEST 90W) HAS PERSISTED NEAR
21.4N 144.9E, APPROXIMATELY 466 NM NORTH OF GUAM. THE SYSTEM IS
CURRENTLY CLASSIFIED AS A SUBTROPICAL STORM/DEPRESSION, GENERALLY
CHARACTERIZED AS HAVING BOTH TROPICAL AND MIDLATITUDE CYCLONE
FEATURES. ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS A
VERY WEAK, DISORGANIZED DISTURBANCE FORMING ALONG THE EASTERN EXTENT
OF THE MONSOON TROUGH. RECENT ASCAT DATA INDICATES A SHARP TROUGH
WITH CONVERGENT SOUTHERLY FLOW OVER THE NORTHERN MARIANAS. UPPER-
LEVEL ANALYSIS INDICATES MARGINAL CONDITIONS (STRONG NORTHERLY
VERTICAL WIND SHEAR (VWS) OF 20-30 KNOTS WITH BROAD DIVERGENCE) NOT
CONDUCIVE FOR TROPICAL CYCLONE DEVELOPMENT, HOWEVER, THIS
DISTURBANCE IS EXPECTED TO DEVELOP QUICKLY AS A SUBTROPICAL CYCLONE
IN RESPONSE TO A DEEP SUBTROPICAL TROUGH. OVER THE NEXT 1 TO 2 DAYS,
INVEST 90W IS FORECASTED TO INTENSIFY AS IT TRACKS NORTHEASTWARD
WITHIN THE DEEP UPPER-LEVEL TROUGH WITH WEAK BAROCLINICITY. AFTER
TAU 48, AS THE SYSTEM TRACKS TOWARD CENTRAL HONSHU, GLOBAL MODELS
INDICATE THE SYSTEM WILL LIKELY TRANSITION TO A WARM-CORE TROPICAL
SYSTEM AS IT TRACKS OVER A WARM POOL OF WATER (SST 28-29C) EAST OF
THE KANTO PLAIN. FOR HAZARDS AND WARNINGS, REFERENCE THE FLEET
WEATHER CENTER SAN DIEGO HIGH WINDS AND SEAS PRODUCT OR REFER TO
LOCAL WMO DESIGNATED FORECAST AUTHORITY. MAXIMUM SUSTAINED SURFACE
WINDS ARE ESTIMATED AT 10 TO 15 KNOTS. MINIMUM SEA LEVEL PRESSURE IS
ESTIMATED TO BE NEAR 1002 MB. THE POTENTIAL FOR THE DEVELOPMENT OF A
SIGNIFICANT TROPICAL CYCLONE WITHIN THE NEXT 24 HOURS IS MEDIUM.

[Hayabusa]

[cycloneye]

Oh boy, the Olympics.

[ElectricStorm]

Next name is Nepartak, which probably should've been retired after the 2016 season...

[aspen]

Next name is Nepartak, which probably should've been retired after the 2016 season...

WPac name retirements usually suck. Some weak storms like Haiku get retired while multi-billion Cat 5s like Nepartak don’t.

Although it seems like this Nepartak really wants to get retired, seeing as it might hit the Olympics.

[ElectricStorm]

Next name is Nepartak, which probably should've been retired after the 2016 season...

WPac name retirements usually suck. Some weak storms like Haiku get retired while multi-billion Cat 5s like Nepartak don’t.

Although it seems like this Nepartak really wants to get retired, seeing as it might hit the Olympics.

It might not be quite as bad since there's no fans but it still could be pretty impactful.

[wxman57]

I started a forecast on it yesterday, taking it inland north of Tokyo on Monday. Tokyo may be in a typhoon watch this weekend, assuming JMA issues such watches. We have TS winds grazing Tokyo for now, but there's a lot of uncertainty. Two days ago, models had it moving into Hokkaido, the island to the north of the main Japanese island, Honshu. Now the models are focusing on a landfall just north of Tokyo.

[cycloneye]

 https://twitter.com/RobertSpetaWX/status/1418335614622179332

[Subtrop]

WTPN21 PGTW 222200
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/TROPICAL CYCLONE FORMATION ALERT (INVEST 90W)//
RMKS/
1. FORMATION OF A SIGNIFICANT TROPICAL CYCLONE IS POSSIBLE WITHIN
120 NM EITHER SIDE OF A LINE FROM 21.5N 147.0E TO 26.0N 150.3E
WITHIN THE NEXT 12 TO 24 HOURS. AVAILABLE DATA DOES NOT JUSTIFY
ISSUANCE OF NUMBERED TROPICAL CYCLONE WARNINGS AT THIS TIME.
WINDS IN THE AREA ARE ESTIMATED TO BE 18 TO 23 KNOTS. METSAT
IMAGERY AT 221800Z INDICATES THAT A CIRCULATION CENTER IS LOCATED
NEAR 21.7N 147.1E. THE SYSTEM IS MOVING EASTWARD AT 10 KNOTS.
2. REMARKS: AN AREA OF CONVECTION (INVEST 90W) PREVIOUSLY LOCATED
NEAR 21.4N 144.9E IS NOW LOCATED NEAR 21.7N 144.7E, APPROXIMATELY
366 NM EAST-SOUTHEAST OF IWO-TO. THE SYSTEM IS CURRENTLY CLASSIFIED
AS A SUBTROPICAL STORM/DEPRESSION, GENERALLY CHARACTERIZED AS HAVING
BOTH TROPICAL AND MIDLATITUDE CYCLONE FEATURES. ANIMATED ENHANCED
INFRARED SATELLITE IMAGERY (EIR) AND A 221852Z SSMIS 91 GHZ
MICROWAVE IMAGE DEPICT A LOW LEVEL CIRCULATION CENTER (LLCC) FORMING
ALONG THE EASTERN EXTENT OF THE MONSOON TROUGH WITH DEEP CONVECTION
OFFSET TO THE EASTERN AND SOUTHEASTERN PERIPHERIES. ENVIRONMENTAL
ANALYSIS INDICATES FAVORABLE CONDITIONS FOR DEVELOPMENT, WITH ROBUST
EQUATORWARD OUTFLOW, LOW (10-15 KTS) VERTICAL WIND SHEAR (VWS), AND
WARM (29-30C) SEA SURFACE TEMPERATURES (SST). THIS DISTURBANCE IS
EXPECTED TO DEVELOP QUICKLY AS A SUBTROPICAL CYCLONE AND IS
FORECASTED TO INTENSIFY AS IT TRACKS NORTHEASTWARD WITHIN A DEEP
UPPER-LEVEL TROUGH WITH WEAK BAROCLINICITY. GLOBAL MODELS INDICATE
THAT AS THE SYSTEM TRACKS TOWARD CENTRAL HONSHU, THE SYSTEM WILL
TRANSITION TO A WARM-CORE TROPICAL SYSTEM EAST OF THE KANTO PLAIN.
WHILE THE SYSTEM IS SUBTROPICAL, REFERENCE THE FLEET WEATHER CENTER
SAN DIEGO HIGH WINDS AND SEAS PRODUCT OR REFER TO THE LOCAL WMO
DESIGNATED FORECAST AUTHORITY FOR HAZARDS AND WARNINGS. MAXIMUM
SUSTAINED SURFACE WINDS ARE ESTIMATED AT 18 TO 23 KNOTS. MINIMUM SEA
LEVEL PRESSURE IS ESTIMATED TO BE NEAR 1004 MB. THE POTENTIAL FOR
THE DEVELOPMENT OF A SIGNIFICANT TROPICAL CYCLONE WITHIN THE NEXT 24
HOURS IS HIGH.
3. THIS ALERT WILL BE REISSUED, UPGRADED TO WARNING OR CANCELLED BY
232200Z.//
NNNN

[Sciencerocks]

[ElectricStorm]

My early prediction is that we'll see something around a 55-60kt TS out of this, with a Cat 1 typhoon not out of the question. Normally I wouldn't be too concerned with this type of system, as Japan is pretty good at handling TC's. However, with the Olympics being there I think this one will be a more serious threat than normal, regardless of intensity.

[Hayabusa]

Tropical Cycloneb
Issued at 2021/07/23 07:25 UTC
Analisys at 07/23 06 UTC
Category TD
Scale -
Intensity -
Center Position N23°25′(23.4°)
E148°30′(148.5°)
Direction and speed of movement NNE 15km/h(7kt)
Central pressure 1000hPa
Maximum wind speed 15m/s(30kt)
Maximum wind gust speed 23m/s(45kt)
Forecast at 07/24 06 UTC
Category TS
Intensity -
Center of probability circle N26°10′(26.2°)
E149°40′(149.7°)
Direction and speed of movement NNE 15km/h(8kt)
Central pressure 998hPa
Maximum wind speed 18m/s(35kt)
Maximum wind gust speed 25m/s(50kt)
Radius of probability circle 150km(80NM)
Forecast at 07/25 06 UTC
Category TS
Intensity -
Center of probability circle N30°5′(30.1°)
E149°25′(149.4°)
Direction and speed of movement N 20km/h(10kt)
Central pressure 996hPa
Maximum wind speed 20m/s(40kt)
Maximum wind gust speed 30m/s(60kt)
Radius of probability circle 240km(130NM)
Forecast at 07/26 06 UTC
Category TS
Intensity -
Center of probability circle N33°50′(33.8°)
E145°25′(145.4°)
Direction and speed of movement NW 25km/h(13kt)
Central pressure 992hPa
Maximum wind speed 23m/s(45kt)
Maximum wind gust speed 35m/s(65kt)
Radius of probability circle 370km(200NM)
Forecast at 07/27 06 UTC
Category TS
Intensity -
Center of probability circle N36°40′(36.7°)
E141°10′(141.2°)
Direction and speed of movement NW 20km/h(11kt)
Central pressure 996hPa
Maximum wind speed 20m/s(40kt)
Maximum wind gust speed 30m/s(60kt)
Radius of probability circle 520km(280NM)
Forecast at 07/28 06 UTC
Category LOW
Intensity -
Center of probability circle N39°5′(39.1°)
E136°40′(136.7°)
Direction and speed of movement NW 20km/h(11kt)
Central pressure 1000hPa
Radius of probability circle 700km(390NM)

[cycloneye]

90W INVEST 210723 1200 24.1N 148.6E WPAC 30 1000

[Hayabusa]

Upgraded to 11W

11W.INVEST

[Hayabusa]

TY2108(Nepartak)
Issued at 2021/07/23 13:30 UTC

Analisys at 07/23 12 UTC
Category TS
Scale -
Intensity -
Center Position N23°55′(23.9°)
E148°20′(148.3°)
Direction and speed of movement N Slow
Central pressure 998hPa
Maximum wind speed 18m/s(35kt)
Maximum wind gust speed 25m/s(50kt)
30-kt wind area E500km(270NM)
W280km(150NM)
Forecast at 07/24 12 UTC
Category TS
Intensity -
Center of probability circle N27°30′(27.5°)
E150°5′(150.1°)
Direction and speed of movement NNE 20km/h(10kt)
Central pressure 998hPa
Maximum wind speed 18m/s(35kt)
Maximum wind gust speed 25m/s(50kt)
Radius of probability circle 120km(65NM)
Forecast at 07/25 12 UTC
Category TS
Intensity -
Center of probability circle N31°40′(31.7°)
E149°20′(149.3°)
Direction and speed of movement N 20km/h(11kt)
Central pressure 996hPa
Maximum wind speed 20m/s(40kt)
Maximum wind gust speed 30m/s(60kt)
Radius of probability circle 220km(120NM)
Forecast at 07/26 12 UTC
Category TS
Intensity -
Center of probability circle N35°30′(35.5°)
E143°50′(143.8°)
Direction and speed of movement NW 30km/h(15kt)
Central pressure 992hPa
Maximum wind speed 23m/s(45kt)
Maximum wind gust speed 35m/s(65kt)
Radius of probability circle 330km(180NM)
Forecast at 07/27 12 UTC
Category TS
Intensity -
Center of probability circle N38°35′(38.6°)
E140°25′(140.4°)
Direction and speed of movement NW 20km/h(10kt)
Central pressure 998hPa
Maximum wind speed 18m/s(35kt)
Maximum wind gust speed 25m/s(50kt)
Radius of probability circle 480km(260NM)
Forecast at 07/28 12 UTC
Category LOW
Intensity -
Center of probability circle N40°40′(40.7°)
E136°5′(136.1°)
Direction and speed of movement WNW 20km/h(10kt)
Central pressure 1000hPa
Maximum wind speed 15m/s(30kt)
Maximum wind gust speed 23m/s(45kt)
Radius of probability circle 650km(360NM)

[Sciencerocks]

[MHC Tracking]

In my opinion, Nepartak is non-tropical, and will likely remain such for a few days. It has a very elongated LLC, not to mention, it has yet to detach from the monsoon trough. This isn't even mentioning the fact that all major models want it to become an extratropical system, not a tropical one. However, if it crosses into the Sea of Japan, several models, including the HWRF and GFS want it to become a subtropical, or potentially even tropical, storm there. With SST's there in the region of 26-27C, that seems like a bona fide possibility.

[StormTracker89]

In my opinion, Nepartak is non-tropical, and will likely remain such for a few days. It has a very elongated LLC, not to mention, it has yet to detach from the monsoon trough. This isn't even mentioning the fact that all major models want it to become an extratropical system, not a tropical one. However, if it crosses into the Sea of Japan, several models, including the HWRF and GFS want it to become a subtropical, or potentially even tropical, storm there. With SST's there in the region of 26-27C, that seems like a bona fide possibility.

Well the good folks at JTWC agree with you, and I believe their assessment is spot on. This system is not tropical, nor will it ever be. However, with the politics of the Olympics involved, no one is going to let it slide by without issuing some sort of warning, as it has the potential to create substantial impacts.

WDPN33 PGTW 231500
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL DEPRESSION 11W (NEPARTAK)
WARNING NR 001//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
INITIAL POSITION: 24.0N 148.6E
INITIAL INTENSITY: 30 KTS
GEOGRAPHIC REFERENCE: 821 NM SOUTHEAST OF YOKOSUKA, JAPAN
MOVEMENT PAST 6 HOURS: NORTH-NORTHEASTWARD AT 08 KTS
SIGNIFICANT WAVE HEIGHT: 10 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS AN AREA
OF EXPANDING DEEP CONVECTION SHEARED TO THE EAST OF A BROAD,
EXPOSED LOW-LEVEL CIRCULATION (LLC). A 230945Z GMI 89GHZ COMPOSITE
MICROWAVE IMAGE REVEALS SHALLOW BANDING WRAPPING LOOSELY AROUND THE
BROAD LLC WITH A WEAKLY DEFINED CENTER; DEEP CONVECTION IS
DISORGANIZED OVER THE EASTERN QUADRANT. THE INITIAL POSITION IS
PLACED WITH MEDIUM CONFIDENCE BASED ON A 231021Z ASCAT-A IMAGE. TD
11W IS CURRENTLY ASSESSED AS A SUBTROPICAL SYSTEM AND IS LOCATED
WITHIN THE SOUTHERN, DIVERGENT REGION OF A DEEP SUBTROPICAL TROUGH
WITH STRONG VENTING TO THE EAST AND SOUTHEAST AND STRONG VERTICAL
WIND SHEAR FUELING STRONG CONVECTIVE BURSTS OVER THE EASTERN AND
SOUTHEASTERN PERIPHERIES. THE INITIAL INTENSITY OF 30 KTS IS
ASSESSED WITH MEDIUM CONFIDENCE BASED ON A 231021Z ASCAT-A IMAGE
SHOWING AN ELONGATED CIRCULATION WITH A HIGHLY ASYMMETRIC WIND
FIELD TYPICAL WITH SUBTROPICAL CYCLONES; THIS IMAGE INDICATES A
SWATH OF CONVERGENT SOUTHERLY WINDS OVER THE EASTERN QUADRANT WITH
30-35 KNOT WINDS AND 10-20 KNOT WINDS OVER THE WESTERN SEMICIRCLE. A
PGTW SUBTROPICAL FIX OF ST1.5 (25-30 KNOTS) ALSO SUPPORTS THE
ASSESSMENT.

INITIAL WIND RADII BASIS: NOT APPLICABLE (THERE ARE NO INITIAL WIND
RADII).

CURRENT STEERING MECHANISM: NORTH-SOUTH ORIENTED SUBTROPICAL RIDGE
(STR)

AGENCY DVORAK AND AUTOMATED FIXES:
PGTW: ST1.5 - 25 KTS
RJTD: T1.5 - 25 KTS

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: UNFAVORABLE
VWS: 25-30 KTS
SST: 29-30 CELSIUS
OUTFLOW: STRONG EQUATORWARD
OTHER FACTORS: SIGNIFICANT DRY AIR ENTRAINMENT

ANALYSIS CONFIDENCE:
INITIAL POSITION: MEDIUM
INITIAL INTENSITY: MEDIUM
INITIAL WIND RADII: NOT APPLICABLE

3. FORECAST REASONING.

SIGNIFICANT FORECAST CHANGES: THIS INITIAL PROGNOSTIC REASONING
MESSAGE ESTABLISHES THE FORECAST PHILOSOPHY. TD 11W IS ASSESSED AS
A SUBTROPICAL CYCLONE.

FORECAST DISCUSSION: TD 11W WILL REMAIN A SUBTROPICAL CYCLONE
THROUGH THE FORECAST PERIOD. IT WILL NOT DEVELOP AS A TYPICAL
TROPICAL CYCLONE WITH CENTRAL, CONSOLIDATING CONVECTION AND A
CONTRACTING, STRENGTHENING WIND FIELD. THIS CRITICAL DISTINCTION
IS IMPORTANT TO KEEP IN MIND AS THE SYSTEM APPROACHES AND MAKES
LANDFALL OVER HONSHU. TD 11W IS CURRENTLY POSITIONED OVER THE
SOUTHERN PERIPHERY OF A DEEP UPPER-LEVEL SUBTROPICAL TROUGH AND IS
FORECAST TO TRACK POLEWARD WITHIN AN AREA OF ENHANCED FLOW BETWEEN
A DEVELOPING UPPER-LEVEL LOW AND THE NORTH-SOUTH ORIENTED STR. TD
11W SHOULD SLOWLY INTENSIFY TO A PEAK OF 50 KNOTS BY TAU 36 TO 48
DUE TO THE STRONG POLEWARD OUTFLOW ASSOCIATED WITH THE DEEPENING
UPPER LOW TO THE WEST. UPPER-LEVEL WINDS ARE EXPECTED TO INCREASE
TO JET STRENGTH ALONG THE EASTERN FLANK OF THIS LOW WITH VWS
INCREASING TO 40-45 KNOTS. CONSEQUENTLY, THE SYSTEM WILL BE UNABLE
TO SUSTAIN ANY TYPE OF CENTRALIZED CONVECTION AND WILL REMAIN
SUBTROPICAL IN NATURE WITH SIGNIFICANT DRY AIR ENTRAINMENT. AFTER
TAU 48, TD 11W WILL TRACK GENERALLY NORTHWESTWARD ALONG THE
SOUTHWEST PERIPHERY OF THE STR WHILE GRADUALLY WEAKENING AS IT
APPROACHES HONSHU DUE TO SLIGHTLY COOLER SST VALUES (26-27C), WEAK
OUTFLOW AND DRY AIR ENTRAINMENT.

MODEL DISCUSSION: DUE TO THE COMPLEX SYNOPTIC STEERING ENVIRONMENT
AND INTERACTION WITH INITIALLY AN UPPER-LEVEL SUBTROPICAL TROUGH
THEN A STRONG UPPER-LEVEL LOW EAST OF HONSHU, THERE IS A LARGE DEGREE
OF UNCERTAINTY IN THE FORECAST TRACK WITH NUMERICAL MODEL GUIDANCE
SHOWING A LARGE SPREAD OF 400-450NM IN THE EXTENDED PERIOD THUS
THERE IS LOW CONFIDENCE IN THE JTWC FORECAST TRACK. DUE TO THE
SUBTROPICAL NATURE OF THIS SYSTEM THROUGH ITS LIFECYCLE, THERE IS
LOW CONFIDENCE IN THE JTWC INTENSITY FORECAST.

FORECAST CONFIDENCE:
TRACK 0 - 72 HR: LOW
TRACK 72-120 HR: LOW
INTENSITY 0 - 72 HR: LOW
INTENSITY 72-120 HR: LOW//
NNNN